A semiconductor integrated circuit is reduced in size by suppressing lateral extension of an impurity region when impurities in the impurity region are thermally diffused in a semiconductor substrate. A second photoresist is formed on an insulation film. The second photoresist is formed to have second openings K2 on both sides of a P-type impurity region so that the second openings K2 partially overlap the P-type impurity region. The insulation film is etched off together with an underlying surface of the semiconductor substrate using the second photoresist as a mask so as to remove the P-type impurity region partially. Then, phosphorus ions (P+) are implanted into the surface of the semiconductor substrate in the etched-off regions using the second photoresist as a mask to form N-type impurity regions that are adjacent the P-type impurity region. After removing the second photoresist, the impurities in the P-type impurity region and the impurities in the N-type impurity region are thermally diffused.