PURPOSE: To improve reliability of a semiconductor device by providing a forming method for an element separation region having uniform film thickness.

CONSTITUTION: Openings 14 and 15 are formed on an oxide preventing film 13 on a substrate 11. Next, while the substrate 11 is being rotated, oxidation inhibiting ions 1 are implanted through openings 14 and 15 into the substrate 11 at an angle θ by which at least one kind of oxidation inhibiting ions 1 of silicon, gallium, or germanium are implanted into the entire surface of the substrate 11 exposed to an opening 14 having a preset opening width W1. This enables the oxidation inhibiting ions 1 to be implanted through each of the openings 14 and 15 into the substrate 11 in a quantity matching the opening widths W1 and W2 of the openings 14 and 15. Through an oxidation treatment of the substrate 1, more uniform element separation regions 16 and 17 having film thicknesses T1 and T2 are formed.