Provided is a method for simply evaluating defects caused in interface states in oxide semiconductor thin films and protective films in TFTs having protective films formed on the surface of oxide semiconductor thin films without actually measuring the characteristics of the same. This evaluation method evaluates defects caused in the interface states by measuring electron states in the oxide semiconductor thin film by a contact method or noncontact method. The defects caused in the interface states are any of the following: (1) threshold value voltage (Vth) when a positive bias is applied to the thin-film transistor, (2) difference in threshold value voltage (ΔVth) before and after applying the positive bias to the thin-film transistor, and (3) threshold value during the first measurement when a plurality of measurements is made of the threshold value voltage when a positive bias is applied to the thin-film transistor.