NOVELTY - Substrate (10) for selenium compound semiconductor, comprises at least a steel base (12) and an aluminum base (14), where: the aluminum base is arranged on one end in a direction of lamination of the steel base and the aluminum base; the steel base is arranged on the other end in the direction of lamination; an alloy layer (20) having a thickness of 0.01-10 mu m is formed between the steel base and the aluminum base; and a thermal oxide film (16) having a thickness of greater than or equal to 6 nm is formed on a surface of the steel base opposite to the aluminum base.    USE - The substrate is useful in: selenium compound semiconductor; and thin film solar cell (claimed).    ADVANTAGE - The substrate: exhibits good insulation characteristics, mechanical strength and flexibility (as the steel base, aluminum base, thermal oxide film and insulation layer that are flexible) even after being exposed to high temperature; is resistant to corrosion; suppresses corrosion by selenium and contamination by dust generated by disintegration of selenium compounds; and allows to form alloy layer having a thickness of 0.01-10 mu m, thus assuring interface adhesion, and suppressing the occurrence of interface peeling and substrate curling.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) producing the substrate for selenium compound semiconductor on which treatment using selenium is performed, comprising: forming a substrate body in which at least a steel base and an aluminum base are laminated so that the aluminum base is arranged on one end in a direction of lamination and the steel base is arranged on the other end in the direction of lamination; and performing thermal oxidation treatment on the substrate body to form a thermal oxide film having a thickness of greater than or equal to 6 nm on a surface of the steel base opposite to the aluminum base, as well as to form the alloy layer between the steel base and the aluminum base;    (2) a thin-film solar cell, comprising the substrate and photoelectric conversion layers formed on the substrate, where the photoelectric conversion layers are selenium compound semiconductor formed by evaporating at least selenium; and    (3) a thin-film solar cell, comprising the substrate, and films laminated on the insulation layer of the substrate and constituting the selenium compound semiconductor, where the films comprises at least a back electrode made of molybdenum and formed on the insulation layer of the substrate, and a photoelectric conversion layer made of copper indium- gallium-selenide and formed on the back electrode.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view of the substrate for selenium compound semiconductors.    Substrate (10)    Steel base (12)    Aluminum base (14)    Oxide film (16)    Alloy layer (20)