The invention provides a MOS semiconductor memory device that achieves excellent data retention characteristics while also achieving high-speed data write performance, low-power operation performance, and high reliability. A MOS semiconductor memory device 601 includes a first insulating film 111 and fifth insulating film 115 having large bandgaps 111a and 115a, a third insulating film 113 having the smallest bandgap 113a, and a second insulating film 112 and fourth insulating film 114 interposed between the third insulating film 113 and the first and fifth insulating films 111 and 115, respectively, and having intermediate bandgaps 112a and 114a.