To provide a method capable of obtaining a thin film, having a resistivity suitable as the light-absorbing layer of a solder cell in a method for forming an Ib-IIIb-VIb2 compound semiconductor (CuInS2) thin film by a CVD method (chemical vapor deposition method), using an organic metallic compound as its raw material.

First, raw gases of three components are introduced in a reaction layer 1 to cause a CuInS2 thin film to phase-grow on a substrate 4, in such a state that the temperature of the substrate 4 is held at a temperature of 440°C. The raw gases of the three components are (H2S/H2) gas, Ar gas containing vapor made of a Cu raw material 71 and Ar gas containing vapor made of an In raw material 81. Then, only the (H2S/H2) gas is introduced in the layer 1 to perform heat treatment on the thin film, in a state such that the temperature of the substrate 4 is held at a temperature of 550°C.