A semiconductor device manufacturing method is a method of forming a semiconductor device that includes a cell part that includes plural transistor cells in each of which a gate of a trench type is formed in a semiconductor layer, and diffused layers are formed on both sides of the gate, and a guard ring part that surrounds the cell part. The semiconductor device manufacturing method includes forming an interlayer dielectric film on a surface of the semiconductor layer in which the gate and the diffused layers are formed; reducing a thickness of the interlayer dielectric film formed in the cell part through etch back; forming a contact part having a shape of a hole or a groove in the interlayer dielectric film at a position above the diffused layer; and forming a metal film on the interlayer dialectic film.