The present invention, without using the process of vacuum or low pressure, or without the use of expensive equipment, can be easily realized by forming the source / drain regions in a self-aligned manner the thin film transistor and a manufacturing method thereof it is an object of the present invention to provide a. One method of manufacturing a thin film transistor of the present invention includes a gate electrode layer 40 disposed over the gate insulating layer 30 is formed on the semiconductor layer 20, to cover the semiconductor layer 20, the semiconductor layer 20 n-type or p-type to form an aliphatic polycarbonate layer 50 having a dopant into the semiconductor layer, and the aliphatic polycarbonate layer forming step, while introducing the dopant into the semiconductor layer 20, a heating step of heating at a temperature to decompose the aliphatic polycarbonate layer 50 and, including the.