A semiconductor device such as a flash memory includes a semiconductor substrate, two gate insulating films formed on the substrate so as to have a first film thickness and a second film thickness smaller than the first film thickness respectively, and a polycrystalline silicon film formed on the gate insulating films so that parts of the polycrystalline silicon film on the respective gate insulating films are on a level with each other and serving as a gate electrode. The substrate is formed with a recess defined by a bottom and sidewalls substantially perpendicular to the bottom, the recess corresponding to the part of the gate insulating film with the first film thickness.