A semiconductor device manufacturing method includes preparing a semiconductor substrate of a first conductivity type, forming a semiconductor layer of the first conductivity type over a main surface of the semiconductor substrate, forming a plurality of first ditches in an upper surface portion of the semiconductor layer such that the first ditches are arranged in a first direction extending along an upper surface of the semiconductor substrate, forming a plurality of second ditches in bottom surface portions of each of the first ditches such that the second ditches are arranged in a second direction perpendicular to the first direction, and covering a side wall of each of the first ditches with a first insulating film and a side wall and a bottom surface of each of the second ditches with a second insulating film thicker than the first insulating film.