TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: In the substrate, an insulation layer made of alumina is formed on a surface of the aluminum base opposite to the steel base, where the insulation layer comprises an anodized film formed by anodizing a material of the aluminum base made of aluminum. The steel base, the aluminum base, the thermal oxide film and the insulation layer are flexible. The thermal oxide film: contains iron(II) oxide, iron(III) oxide and chromium oxide; and is an iron-based oxide film containing iron(III) oxide as a principal component. The thermal oxide film has a thickness of at least 25 nm. The steel base comprises ferritic stainless steel or austenitic stainless steel. Preferred Method: The method further comprises forming the insulation layer made of alumina on the surface of the aluminum base opposite to the steel base. The thermal oxidation treatment is performed after the step of forming the insulation layer and before the treatment using selenium. The insulation layer is formed by anodization treatment. The thermal oxide film is formed: at 150-600 degrees C; and in an atmosphere containing oxygen, an atmosphere containing carbon dioxide gas, or an atmosphere containing water vapor. The back electrode is formed by using a roll-to-roll process, where an integrated device comprises the back electrode formed in a predetermined pattern and the photoelectric conversion layer formed in a predetermined pattern. The thin-film solar cell further comprises a soda lime glass layer formed between the insulation layer of the substrate and the back electrode. EXAMPLE - Aluminum base formed with a purity of 4N was formed on a steel base of SUS30 ferritic stainless steel. The substrate body was formed by pressurizing and bonding of the aluminum base to the steel base. Then a protective sheet was pasted onto the rear surface and side surfaces of the steel base so that the steel base does not directly contact the solution. The substrate body was immersed together with an aluminum opposing electrode into a solution of 0.1 M oxalic acid adjusted to temperature of 16 degrees C and then the DC voltage of 40 V was applied for 1 hour, causing anodization of the aluminum base of the substrate body, to form an anodized film with a thickness of 10 mu m as an insulation layer. The substrate body on which the anodized film had been formed was washed with pure water and the protective sheet was removed, and was additionally washed in acetone and ethanol, and the components of the adhesive of the protective sheet were removed. Then the substrate body was subjected to thermal oxidation treatment in a heating furnace for 1 hour at 300 degrees C in a dry air atmosphere, to obtain the substrate with an oxide film of thickness 6 mu m formed on the back surface of a steel base. Then on the front surface of the insulation layer of the substrate, a soda lime glass layer was formed at a thickness of 200 nm by sputtering, and on the surface of the soda lime glass layer, a molybdenum film of thickness 500 nm was formed as the back electrodes. Then a copper indium-gallium-selenide layer was formed on the front surface of the molybdenum film at a thickness of 1.5-2.5 mu m as a photoelectric conversion layer using a multisource vapor deposition apparatus at 520 degrees C by simultaneous vapor deposition of copper and selenium, followed by simultaneous vapor deposition of indium, gallium and selenium.