PURPOSE: To prevent a base from contaminating with metal from metal- containing mask layer by ion implanting the base through a metallic atom implantation preventive layer from a mask due to the ion implantation between the base including the ion implanted region and the metal containing mask layer.

CONSTITUTION: A metallic atom implantation preventive layer 4 made of SiO2 or the like is formed on a semiconductor substrate 1 made of Si, GaAs, etc. to be selectively ion implanted. A metal containing mask layer 2 made of high melting point metal W, Mo, Ti, etc. covers the whole layer 4, coated with a photoresist film, an opening 8 is formed on a region 3 to be ion implanted of the substrate 1 by pattern exposure and development to form an upper layer mask 7, and an opening 5 is opened at the layer 2 through the opening 8 of the mask 7. With the layer 2 as a main mask impurity ions P+, B+, As+, etc., are implanted through the openings 8, 5 and the layer 4. Thus, it can prevent the substrate from contaminating with the metal from the mask layer.