In film forming of thin film semiconductors (TFTs), a gate electrode having an anodic-oxidizable material is formed on a substrate, and the surface of the gate electrode is oxidized by anodic oxidation in an electrolytic solution so that the surface of the gate electrode is coated with an insulating film. The doping is performed using the gate electrode and the anodic oxide film as a mask, to form a source and a drain region. Then, when the laminate is again dipped in an electrolytic solution, and a voltage is applied to the gate electrode so that a current curing produces in the laminate. During the current curing, a positive voltage is preferably applied to the gate electrode for N-channel TFTs and a negative voltage is preferably to the gate electrode for P-channel TFTs. After the doping, the source and the drain region is activated by laser annealing or the like, prior to the current curing.