A semiconductor device includes a semiconductor substrate, and nonvolatile memory cells, each of the cells including a channel region having a channel length and a channel width, a tunnel insulating film, a floating gate electrode, a control gate electrode, an inter-electrode insulating film between the floating and control gate electrodes, and an electrode side-wall insulating film on side-wall surfaces of the floating and control gate electrodes, the electrode side-wall insulating film including first and second insulating films having first and second dielectric constants, the first dielectric constant being higher than the second dielectric constant, the second dielectric constant being higher than a dielectric constant of a silicon nitride film, the first insulating film being in a central region of a facing region between the floating and control gate electrodes, the second insulating region being in the both end regions of the facing region and protruding from the both end portions.