A technique of suppressing the potential crowding in the vicinity of the outer periphery of a bottom face of a trench without ion implantation of a p-type impurity is provided. A method of manufacturing a semiconductor device having a trench gate structure comprises an n-type semiconductor region forming process. In the n-type semiconductor region forming process, a p-type impurity diffusion region in which a p-type impurity contained in a p-type semiconductor layer is diffused is formed in at least part of an n-type semiconductor layer that is located below an n-type semiconductor region.