PURPOSE: To lower the influence of the mobile ions on the surface by forming a second conductivity of region wider in width, shallower in depth, and higher in impurity concentration, on a second conductivity of resistance region.

CONSTITUTION: A mask is made of silicon oxide on the surface of a substrate 1, and for the opening of the mask, a p region where the concentration of the surface impurities is on a level of 1018atoms/cm3 is made in the depth of the 2-3μm by heat diffusion of boronic ions after implantation. Next, the opening of the mask is widened, and the same ions are implanted with high dosage, and then a p-type preventive layer 6, which is a quarter as deep as the p region 2 or less but in which the concentration of impurities at the surface is on a level of 1019atom/cm3 is made by a short time of thermal diffusion. Since the p+-preventive layer 6 is wider than the p region 2, it covers a low-resistance area 2 surely, whereby the influence of the mobile ions on the surface can be lowered to one tenth of conventional structure.