A semiconductor device comprising: a semiconductor layer having a structure in which a first conductivity type drain layer, a second conductivity type channel layer, and a first conductivity type source layer are layered in the stated order, the source layer being exposed at a surface of the semiconductor layer; a gate trench that passes through the source layer and through the channel layer from the surface of the semiconductor layer, a deepest part of the gate trench reaching the drain layer; a gate insulating film formed conformal to an inner surface of the gate trench and the surface of the semiconductor layer; a gate electrode embedded inside the gate trench interposed by the gate insulating film; and a part of the gate insulating film in contact with the surface of the semiconductor layer being thicker than a part of the gate insulating film in contact with the channel layer at a side surface of the gate trench, wherein the gate trench includes an upper edge that is formed at the opening end and that has an oblique surface continuous with the surface of the semiconductor layer as a part of the side surface, and the gate insulating film includes an overhang portion that projects to an inside of the gate trench in the upper edge, wherein the overhang portion has a circular shape swelling toward the inside of the gate trench when cross-sectionally viewed in a cutting plane that crosses the gate trench in a width direction, wherein the gate electrode has a constricted portion that is selectively concaved in a circular shape along the overhang portion when cross-sectionally viewed in the cutting plane, wherein the gate electrode is made of polysilicon, wherein the gate electrode has an extension portion extending upwardly from the surface of the semiconductor layer, and wherein an upper surface of the extension portion is positioned at a place within the extent of the part of the gate insulating film in a thickness direction of the part of the gate insulating film in contact with the surface of the semiconductor layer, characterized in that the semiconductor layer is made of silicon carbide, wherein the gate insulating film is integrally made of silicon oxide, and integrally includes a bottom insulating film on the bottom surface of the gate trench, a side insulating film on the side surface and forming the part of the gate insulating film in contact with the channel layer at a side surface of the gate trench, the overhang portion at the upper edge of the gate trench, and a plane insulating film on and in contact with the surface of the semiconductor layer. a semiconductor layer having a structure in which a first conductivity type drain layer, a second conductivity type channel layer, and a first conductivity type source layer are layered in the stated order, the source layer being exposed at a surface of the semiconductor layer; a gate trench that passes through the source layer and through the channel layer from the surface of the semiconductor layer, a deepest part of the gate trench reaching the drain layer; a gate insulating film formed conformal to an inner surface of the gate trench and the surface of the semiconductor layer; a gate electrode embedded inside the gate trench interposed by the gate insulating film; and a part of the gate insulating film in contact with the surface of the semiconductor layer being thicker than a part of the gate insulating film in contact with the channel layer at a side surface of the gate trench, wherein the gate trench includes an upper edge that is formed at the opening end and that has an oblique surface continuous with the surface of the semiconductor layer as a part of the side surface, and the gate insulating film includes an overhang portion that projects to an inside of the gate trench in the upper edge, wherein the gate electrode has a constricted portion that is selectively concaved in a circular shape along the overhang portion when cross-sectionally viewed in the cutting plane, wherein the gate electrode is made of polysilicon, wherein the gate electrode has an extension portion extending upwardly from the surface of the semiconductor layer, and wherein an upper surface of the extension portion is positioned at a place within the extent of the part of the gate insulating film in a thickness direction of the part of the gate insulating film in contact with the surface of the semiconductor layer, characterized in that the semiconductor layer is made of silicon carbide, wherein the gate insulating film is integrally made of silicon oxide, and integrally includes a bottom insulating film on the bottom surface of the gate trench, a side insulating film on the side surface and forming the part of the gate insulating film in contact with the channel layer at a side surface of the gate trench, the overhang portion at the upper edge of the gate trench, and a plane insulating film on and in contact with the surface of the semiconductor layer.