In a semiconductor device manufacturing method, an insulating layer is formed on a front surface of a semiconductor substrate. Trenches are formed in the substrate by using the insulating layer as a mask so that a first portion of the insulating layer is located on the front surface between the trenches and that a second portion of the insulating layer is located on the front surface at a position other than between the trenches. The entire first portion is removed, and the second portion around an opening of each trench is removed. The trenches are filled with an epitaxial layer by epitaxially growing the epitaxial layer over the front surface side. The front surface side is polished by using the remaining second portion as a polishing stopper.