A method for fabricating a chalcopyrite-type thin film solar cell includes a first step of forming onto a Mo electrode layer 2 a precursor including an In metal layer and a Cu—Ga alloy layer by sputtering, a second step of attaching an alkali-metal solution onto the precursor, a selenization step of subjecting the substrate 1 which has undergone both the first and the second steps to a selenization treatment, and a transparent electrode forming step of depositing an optically transparent conductive layer. As the alkali-metal solution, an aqueous solution of an alkali metal compound, such as sodium tetraborate, sodium sulfide, and sodium aluminum sulfate, can be used.