PURPOSE: To restrain a thick field oxide from biting, in bird's beak, into an element forming region from the end part of an antioxidant mask, i.e., silicon nitride, when the isolation of an integrated circuit is effected by forming the field oxide selectively.

CONSTITUTION: After deposition of silicon oxide 2 on a semiconductor substrate 1, silicon nitride 3 is deposited on the silicon oxide 2 in an element forming region 8. Subsequently, a field antireversion layer 4 is formed in an isolation region 7 through ion implantation followed by ion implantation 32 for rendering the silicon amorphous in the isolation region 7 using the silicon nitride 3 as a mask. In this regard, the field oxide 6 is restrained from biting, in bird's beak, into the element forming region 8 because the amorphous silicon has higher oxidation rate than single crystal silicon and the oxidation is proceeds selectively in the vertical direction.