Variations in height of a top of an element isolation region, which is embedded in a trench surrounding the periphery of a fin having a channel region of a split-gate MONOS memory, are suppressed to improve reliability of a semiconductor device. An element isolation region embedded in a trench between a plurality of fins, which are part of a semiconductor substrate in a memory cell region and protrude above the semiconductor substrate, is comprised of an insulating film covering the bottom of the trench and a silicon nitride film covering the top of the insulating film.