An object of the invention is to prevent, in the flux method, diffusion of substances that constitute the atmosphere of the outer vessel into the reactor. The apparatus for producing a group III nitride based compound semiconductor, the apparatus including a reactor which maintains a group III metal and a metal differing from the group III metal in a molten state, a heating apparatus for heating the reactor, and an outer vessel for accommodating the reactor and the heating apparatus, characterized in that diffusion of substances that constitute the atmosphere of the outer vessel into the reactor is prevented.