A semiconductor integrated circuit is reduced in size by suppressing lateral extension of an isolation region when impurities are thermally diffused in a semiconductor substrate to form the isolation region. Boron ions (B+) are implanted into an epitaxial layer through a third opening K3 to form a P-type impurity region, using a third photoresist as a mask. Then a fourth photoresist is formed on a silicon oxide film to have fourth openings K4 (phosphorus ion implantation regions) that partially overlap the P-type impurity region. Phosphorus ions (P+) are implanted into the surface of the epitaxial layer in etched-off regions using the fourth photoresist as a mask to form N-type impurity regions that are adjacent the P-type impurity region. After that, a P-type upper isolation region is formed in the epitaxial layer by thermal diffusion so that the upper isolation region and a lower isolation region are combined together to make an isolation region.