of all as shown in fig2 a an element separation film is formed on a silicon semiconductor substrate 9 as required and a gate insulation film 10 is formed to a thickness of 200 700 u00e5 by for example thermal oxidation of the silicon semiconductor substrate after that a tunnel drain 11 constituting a drain section of a cell transistor of the eeprom cell is formed by for example an ion injection method a tunnel window section is formed using a well known technique and then a tunnel insulation film 12 is formed to a thickness of 15 120 u00e5 by for example thermal oxidation next as shown in fig2 b nitriding is carried out for the gate insulation film at the same time as for the tunnel insulating film as a result of this processing a boundary level trap and unbonded crystal atoms of the boundary of the tunnel insulation film and the gate insulation film are filled with nitrogen and the resistance to overwriting resistance to hot electrons and reliability of each of the insulation films is improved normally this process is carried out at an extremely high temperature using rta rapid thermal annealing such as ramp annealing for example processing.