NOVELTY - Substrate comprises an oxide film having insulating properties and a thickness of more than 1-30 mu m on a surface of an aluminum alloy containing 2-7 wt.% of magnesium, and the balance of aluminum and inevitable impurities.    USE - The substrate is useful: in solar cell or thin film solar cell (claimed); as a thin-film solar cell substrate; and in a printed wiring board.    ADVANTAGE - The substrate: exhibits excellent insulating performance and high strength at elevated temperatures; withstands voltage characteristics; and efficiently produces flexible solar cells.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) a solar cell substrate comprising the aluminum alloy and the insulating layer (3) formed on a surface of the aluminum alloy;    (2) the solar cell substrate obtained by a process comprising: forming a porous anodized film with a thickness of 5-30 mu m by anodizing the aluminum alloy in an aqueous sulfuric or oxalic acid solution; and sealing the porous anodized film in an aqueous boric acid solution containing sodium borate;    (3) a thin-film solar cell (1) comprising the substrate and a light-absorbing layer (5) formed on the substrate, with a backside electrode layer (4) interposed between the substrate and the light-absorbing layer; and    (4) a method of manufacturing the thin-film solar cell, comprising: continuously feeding the aluminum alloy plate, which is wound into a roll; subjecting the fed aluminum alloy plate (2) to anodizing treatment, rinsing with water, sealing treatment, rinsing with water and drying; winding the aluminum alloy plate having been dried into a roll; and continuously feeding the aluminum alloy plate having been wound into the roll to form a backside electrode layer and a light-absorbing layer.    DESCRIPTION OF DRAWING(S) - The figure shows the cross-sectional view of a thin-film solar cell.    Thin-film solar cell (1)    Aluminum alloy plate (2)    Insulating layer (3)    Backside electrode layer (4)    Light-absorbing layer (5)