A semiconductor device includes semiconductor substrate, isolation insulating film, nonvolatile memory cells, each of the cells including tunnel insulating film, FG electrode, CG electrode, interelectrode insulating film between the CG and FG electrodes and including a first insulating film and a second insulating film on the first insulating film and having higher permittivity than the first insulating film, the interelectrode insulating film being provided on a side wall of the floating gate electrode in a cross-section view of a channel width direction of the cell, thickness of the interelectrode insulating film increasing from an upper portion of the side wall toward a lower portion of the side wall, thickness of the second insulating film on an upper corner of the FG electrode being thicker than thickness of the second insulating film on the other portions of the side wall in the cross-section view of the channel width direction.