A thin film semiconductor device comprises a substrate, a gate electrode disposed above the substrate, an oxide semiconductor layer disposed above the substrate so as to oppose the gate electrode, a channel protective layer disposed on the oxide semiconductor layer, and a source electrode and a drain electrode each connected to the oxide semiconductor layer. The density of states DOS [eV−1cm−3] of oxygen defects in the oxide semiconductor layer satisfies the following relationship: DOS≦1.710×1017×(Ec−E)2−6.468×1017×(Ec−E)+6.113×1017 provided that 2.0 eV≦Ec−E≦2.7 eV where Ec [eV] is an energy level of a conduction band edge of the oxide semiconductor layer and E [eV] is a predetermined energy level of the oxide semiconductor layer.