PURPOSE: To make thick films even on small element separation regions and improve ability in electric separation of elements by performing a process of implantation of pro-oxidant to implant pro-oxidant according to the sizes of the element separation regions.

CONSTITUTION: A silicon oxide film 12, a polycrystalline silicon film 13, and a silicon nitride film 14 are formed on a silicon substrate 11, a mask pattern 14a is formed from the silicon nitride film 14, and ions are implanted with the pattern used as a mask to form channel stop baths 15-1 and 15-2. Ions of BF2 as pro-oxidant are implanted selectively into the polycrystalline silicon film 13 in an element separation region B-11 with the mask pattern 14a used as a mask. Implantation conditions are determined based on the thickness of the polycrystalline silicon film 13, the sizes of the element separation regions B-11 and B-12, and the relations therebetween so that BF2 may effectively function as pro-oxidant. Silicon oxide films 16-1 and 16-2 as element separation oxide films are formed by thermal oxidation. The mask pattern 14a, the polycrystalline silicon film 13, and the silicon oxide film 12 are removed by etching.