A method for fabricating a thin-film semiconductor device according to the present disclosure includes: preparing a glass substrate; forming, above the glass substrate, an undercoat layer including a nitride film; forming a barrier layer above the undercoat layer; forming a molybdenum metal layer above the barrier layer; forming a gate electrode from the molybdenum metal layer; forming a gate insulating film above the gate electrode; forming a non-crystalline silicon layer as a non-crystalline semiconductor layer above the gate insulating film; forming a polycrystalline semiconductor layer including a polysilicon layer by annealing the non-crystalline silicon layer using a continuous-wave (CW) laser, the non-crystalline silicon layer being crystallized by the annealing; and forming a source electrode and a drain electrode above the polysilicon layer. Part of the barrier layer changes into a layer including oxygen atoms as a major component by the annealing when forming the polysilicon layer.