A semiconductor device having improved reliability is disclosed. In a semiconductor device according to one embodiment, an element isolation region extending in an X direction has a crossing region that crosses, in plan view, a memory gate electrode extending in a Y direction that intersects with the X direction at right angles. In this case, in the crossing region, a width in the Y direction of one edge side, the one edge side being near to a source region, is larger than a width in the Y direction of the other edge side, the other edge side being near to a control gate electrode.