PURPOSE: To prevent unnecessary substances from mixing into an amorphous semiconductor layer during deposition of the amorphous semiconductor layer.

CONSTITUTION: Before each amorphous silicon layer is deposited on a conductive substrate 11, a material gas or a main material gas for deposition of each amorphous silicon type is supplied into a deposition chamber 101 from a gas supply unit 200. After pressure in the deposition chamber 101 stabilizes to a predetermined pressure, a microwave energy is introduced from a microwave power source (not illustrated) via a waveguide 112 and a dielectric window 113 into the deposition chamber 101 to cause plasma discharge, thereby depositing a thin film of 0.1-10μm thickness over the inner wall surface of the deposition chamber 101.