1 . A state display apparatus, comprising: a receiving unit which receives a radio signal transmitted by each of a plurality of radio transmission apparatuses, the radio signal indicating that a change has occurred in a state of a management object associated with a radio transmission apparatus that is a transmission source of the radio signal; a display unit which displays a current state of each management object and which displays a margin of communication by the radio signal between the state display apparatus and each radio transmission apparatus; and a control unit which, based on a radio signal received by the receiving unit, specifies a management object for which a change in a state thereof is notified by the radio signal and a state of the management object after the change, which causes the display unit to display the specified state after the change as a current state of the specified management object, which obtains, from a reception strength of a radio signal received by the receiving unit, a margin of communication by the radio signal between the state display apparatus and a radio transmission apparatus that is a transmission source of the radio signal, and which causes the display unit to display the obtained margin of communication.
2 . The state display apparatus according to claim 1 , wherein the display unit includes a plurality of light emitting element pairs respectively associated with different management objects, each of the plurality of light emitting element pairs includes a first light emitting element and a second light emitting element, and the control unit causes the first light emitting element of the light emitting element pair associated with the specified management object to operate in an operational state in accordance with the specified state after the change, and causes the second light emitting element of the light emitting element pair associated with the specified management object to operate in an operational state in accordance with the obtained margin of communication.
3 . The state display apparatus according to claim 1 , wherein the display unit includes a plurality of light emitting elements respectively associated with different management objects, and the control unit causes the light emitting element associated with the specified management object to operate in an operational state in accordance with a combination of the specified state after the change and the obtained margin of communication.
4 . The state display apparatus according to claim 3 , wherein each of the plurality of light emitting elements is a color light emitting element, and the control unit causes the light emitting element associated with the specified management object to emit light in a color in accordance with a combination of the specified state after the change and the obtained margin of communication.
5 . The state display apparatus according to claim 1 , further comprising an instruction accepting unit which accepts a history display instruction, wherein the control unit: stores, each time a radio signal received by the receiving unit is processed, history information indicating a state of the management object and a margin of communication by the radio signal displayed by the display unit, in a storage apparatus; and when a history display instruction is accepted by the instruction accepting unit, causes the display unit to display, based on history information stored in the storage apparatus, a state of each management object and a margin of communication by the radio signal between the state display apparatus and each radio transmission apparatus that have been previously displayed on the display unit.
6 . The state display apparatus according to claim 5 , wherein the control unit additionally stores the history information in the storage apparatus in a form which enables an order of storage in the storage apparatus to be understood, the instruction accepting unit accepts a history display instruction designating, in an order of storage in the storage apparatus, history information of each management object to be used to display history, and when a history display instruction is accepted by the instruction accepting unit, the control unit causes the display unit to display, based on history information of each management object for which an order of storage in the storage apparatus matches an order designated by the history display instruction, a state of each management object and a margin of communication by the radio signal between the state display apparatus and each radio transmission apparatus that have been previously displayed on the display unit.
7 . The state display apparatus according to claim 1 , further comprising an instruction accepting unit which accepts a history display instruction accompanied by a designation of a management object to be a display object of history, wherein the control unit: stores, each time a radio signal received by the receiving unit is processed, history information indicating a state of the management object and a margin of communication by the radio signal displayed by the display unit, in a storage apparatus in a form which enables an order of storage in the storage apparatus to be understood; and when a history display instruction is accepted by the instruction accepting unit, causes the display unit to display, based on a plurality of pieces of latest history information with respect to a management object designated in the history display instruction, a state of a management object and a margin of communication indicated by each of the plurality of pieces of history information.
8 . A state display apparatus, comprising: a receiving unit which receives a radio signal transmitted by each of a plurality of radio transmission apparatuses, the radio signal indicating that a change has occurred in a state of a management object associated with a radio transmission apparatus that is a transmission source of the radio signal; an accepting unit which accepts a designation of a management object to be a display object of a margin of communication by the radio signal; a display unit which displays a current state of each management object and which displays a margin of communication by the radio signal between the state display apparatus and the management object designated as a display object of a margin of communication; and a control unit which, based on a radio signal received by the receiving unit, specifies a management object for which a change in a state thereof is notified by the radio signal and a state of the management object after the change, which causes the display unit to display the specified state after the change as a current state of the specified management object and, when the specified management object is a management object designated as the display object of a margin of communication, further obtains a margin of communication by the radio signal from a reception strength of a radio signal received by the receiving unit and causes the display unit to display the obtained margin of communication.
9 . The state display apparatus according to claim 8 , wherein the display unit includes a segment display, and the control unit causes a numerical value indicating a margin of communication by the radio signal to be displayed on the segment display of the display unit when the specified management object is a management object designated as a display object of a margin of communication.
10 . The state display apparatus according to claim 8 , wherein the display unit includes a light emitting element, and the control unit causes the light emitting element of the display unit to blink in a pattern in accordance with a margin of communication by the radio signal when the specified management object is a management object designated as a display object of a margin of communication.
11 . The state display apparatus according to claim 8 , wherein the display unit includes a plurality of light emitting elements which display a margin of communication by the radio signal, and the control unit changes the number of light emitting elements to be lighted among the plurality of light emitting elements of the display unit in accordance with a margin of communication by the radio signal when the specified management object is a management object designated as a display object of a margin of communication.
12 . The state display apparatus according to claim 8 , wherein a radio signal received by the receiving unit includes a device ID of a radio transmission apparatus having transmitted the radio signal, and the control unit: holds correspondence relationship information indicating a correspondence relationship between each management object and a device ID of one or more radio transmission apparatuses transmitting a radio signal indicating that a state of each management object has changed; stores, each time a radio signal is received by the receiving unit, a margin of communication obtained from a reception strength of the received radio signal in association with a device ID included in the radio signal, in a storage apparatus; and when a management object of which a state is notified to have changed by a radio signal received by the receiving unit is a management object designated as a display object of a margin of communication, specifies, based on the correspondence relationship information, a device ID of each radio transmission apparatus which transmits a radio signal indicating that the state of the management object has changed and which is not a transmission source of a current radio signal, and causes the display unit to display a margin of communication stored in the storage apparatus in association with each specified device ID and a margin of communication obtained from a reception strength of the current radio signal.
13 . A state display system, comprising: the state display apparatus according to claim 1 ; and a plurality of radio transmission apparatuses which respectively generate power from an operating force of a user and transmit the radio signal using generated power.
14 . A state display system, comprising: the state display apparatus according to claim 1 , further including a transmitting unit which transmits a second radio signal which indicates a margin of communication obtained from a reception strength of a radio signal received by the receiving unit and which is addressed to a radio transmission apparatus that is a transmission source of the radio signal; and a plurality of radio transmission apparatuses each transmitting the radio signal, and including a radio transmission apparatus-side receiving unit which receives the second radio signal addressed to its own radio transmission apparatus from the state display apparatus, a display unit which displays a margin of communication, and a control unit which causes the display unit to display a margin of communication based on the second radio signal which is addressed to its own radio transmission apparatus and which is received by the radio transmission apparatus-side receiving unit.
15 . A radio transmission apparatus, comprising: a radio transmission apparatus-side transmitting unit which transmits a radio signal indicating that a state of a management object has changed; a radio transmission apparatus-side receiving unit which receives a second radio signal that is transmitted by a state display apparatus having received a radio signal transmitted by the radio transmission apparatus-side transmitting unit, the second radio signal being addressed to its own radio transmission apparatus and indicating a margin of communication by the radio signal; a display unit which displays a margin of communication; and a control unit which causes the display unit to display a margin of communication based on a second radio signal, the second radio signal being addressed to its own radio transmission apparatus and having been received by the radio transmission apparatus-side receiving unit.
