A semiconductor device including a source region formed at one main face of a semiconductor substrate; a drain region formed at the one main face and connected to the source region through a channel region; a gate electrode formed above the channel region; a drift layer formed at the one main face at a position between a lower portion of the gate electrode and the drain region; a trench including an opening in which one end is at the lower portion of the gate electrode and another end is at a position adjacent to the drain region, the trench being formed in the semiconductor substrate at a predetermined depth from the one main face to cut vertically across the drift layer; and an electrical field weakening portion, provided at vicinity of the one end, that weaken an electrical field generated between the source region and the drain region.