In an embodiment, an insulating film is formed over a flat surface; a mask is formed over the insulating film; a slimming process is performed on the mask; an etching process is performed on the insulating film using the mask; a conductive film covering the insulating film is formed; a polishing process is performed on the conductive film and the insulating film, so that the conductive film and the insulating film have equal thicknesses; the conductive film is etched, so that a source electrode and a drain electrode which are thinner than the conductive film are formed; an oxide semiconductor film is formed in contact with the insulating film, the source electrode, and the drain electrode; a gate insulating film covering the oxide semiconductor film is formed; and a gate electrode is formed in a region which is over the gate insulating film and overlaps with the insulating film.