In a transistor, a drain electrode to which a high electric field is applied is formed over a flat surface, and an end portion of a gate electrode on the drain electrode side in a channel width direction and an end portion of the gate electrode in a channel length direction are covered with an oxide semiconductor with a gate insulating layer between the gate electrode and the oxide semiconductor layer, so that withstand voltage of the transistor is improved. Further, a semiconductor device for high power application, in which the transistor is used, can be provided.