PURPOSE: To improve data-retaining properties by making thinner a gate insulating film between a floating gate and a control gate and a gate insulating film at a charge-accumulating part of MNOS type constituting EPROMs when constituting and IC with an EPROM and a MNOS type EPROM.

CONSTITUTION: An involatile EPROM consists of a FETQEM where a control gate is provided on a floating gate through a gate insulating film. Also, an EEPROM consists of a FETQFM where a floating gate is provided on a tunnel insulating film and a MISFETQFS for selecting memory for driving it. Further, an EPROM consists of a FETQMM where a gate electrode is provided on a gate insulating film with a charge-accumulating part and a MISFETQMS for selecting memory cell for driving it. Thus, when constituting as in the above, a first gate insulating film 101 is formed by allowing a substrate 1 to be subjected to thermal oxidation and a second gate insulating film 104 consists of a mixed film where SiO2, Si3N4, and SiO2 are laminated in sequence from the lower layer.