There is provided a method for manufacturing a flexible semiconductor device. The method of the flexible semiconductor device according to the present invention comprises the steps of: (i) forming an insulating layer on one of principal surfaces of a metal foil; (ii) forming a semiconductor layer on the insulating layer, and then forming source and drain electrodes so that the source and drain electrodes contact with the semiconductor layer; (iii) forming a flexible film layer so that the flexible film layer covers the semiconductor layer and the source and drain electrodes; (iv) forming vias in the flexible film layer, and thereby a semiconductor device precursor is provided; and (v) subjecting the metal foil to a processing treatment, and thereby forming a gate electrode from the metal foil, wherein, in the step (v) of the processing treatment of the metal foil, the gate electrode is formed in a predetermined position by using at least one of the vias of the semiconductor device precursor as an alignment marker.