When an oxide semiconductor film is microfabricated to have an island shape, with the use of a hard mask, unevenness of an end portion of the oxide semiconductor film can be suppressed. Specifically, a hard mask is formed over the oxide semiconductor film, a resist is formed over the hard mask, light exposure is performed to form a resist mask, the hard mask is processed using the resist mask as a mask, the oxide semiconductor film is processed using the processed hard mask as a mask, the resist mask and the processed hard mask are removed, a source electrode and a drain electrode are formed in contact with the processed oxide semiconductor film, a gate insulating film is formed over the source electrode and the drain electrode, and a gate electrode is formed over the gate insulating film, the gate electrode overlapping with the oxide semiconductor film.