Provided is a semiconductor device formed with a trench portion for providing a concave portion having a continually varying depth in a gate width direction and with a gate electrode provided within the trench portion and on a top surface thereof via a gate insulating film. Before the formation of the gate electrode, an impurity is added to at least a part of the source region and the drain region by ion implantation from an inner wall of the trench portion, and then heat treatment is performed for diffusion and activation to form a diffusion region from the surface of the trench portion down to a bottom portion thereof. Current flowing through a top surface of the concave portion of the gate electrode at high concentration can flow uniformly through the entire trench portion.