An isolating/insulating film is formed on the surface of a p.sup.- silicon substrate in an element isolating region. An nMOS transistor having a pair of n-type source/drain regions is formed within an element forming region isolated by the isolating oxide film. A p.sup.+ impurity diffusion region is formed on the p.sup.- silicon substrate in such a manner as to be contacted with the lower surface of the isolating oxide film in the element isolating region and to extend at a specified depth from the surface of the p.sup.- silicon substrate in the element forming region. A p-type impurity diffusion region having a p-type impurity concentration higher than that of the p.sup.- silicon substrate is formed at the side end portion of the isolating oxide film in such a manner as to be contacted with the n-type source/drain region. With this arrangement, it is possible to reduce leakage current caused by the distribution of crystal defects in a depletion layer.