An improvement is achieved in the reliability of a semiconductor device. In a memory cell region, a plurality of fins are provided which are portions of a semiconductor substrate extending in an x-direction along a main surface of the semiconductor substrate and spaced apart from each other in a y-direction orthogonal to the x-direction along the main surface of the semiconductor substrate. Between the fins adjacent to each other in the y-direction, a portion of an upper surface of an isolation region is at a position higher than a surface obtained by connecting a position of the upper surface of the isolation region which is in contact with a side wall of one of the fins to a position of the upper surface of the isolation region which is in contact with a side wall of the other fin. In a cross section along the y-direction, the upper surface of the isolation region has a projecting shape.