PURPOSE: To improve the dielectric strength of a second gate oxide film and a memory holding characteristic by carrying out thermal treatment in an inactive gas containing a little amount of oxygen to form a very thin oxide film on the surface of a first non-crystalline silicon film and preventing evaporation of impurities.

CONSTITUTION: A field oxide film 22 and a first thermal oxide film 23 are formed on the surface of a P- type silicon substrate 21. A first polycrystalline silicon film 24 is deposited and annealed in Ar gas having O2 of 0.005% concentration. Furthermore, the gas is replaced by a mixed gas having the ratio of Ar to O2 equal to 1:1 and the film 24 is subjected to thermal oxidation therein to form a second thermal oxide film 25. A second polycrystalline silicon film 26 is deposited on the film 25 and subjected to patterning using a mask 27 to form a floating gate 29, a second gate oxide film 30 and a control gate 31. After As+ ion is injected into the control gate 31 and subjected to thermal oxidation, an oxide film 32 and N+ type source and drain regions 33 and 34 are formed and a PSG film 35, a source electrode 37 and a drain electrode 38 are formed, thereby to obtain an EPROM cell.