PURPOSE: To prevent lowering of junction breakdown strength of an N-type diffusion layer by making concentration of a field dope layer low at a part wherein the field dope layer of an isolation region is in contact with the N-type diffusion layer.

CONSTITUTION: A P-well 4 and an N-well 6 are formed on a surface of a P-type silicon substrate 2, enclosed with a field oxide film 8 and isolated. In the P-well 4, boron is introduced as P-type impurities below the field oxide film 8 and a field dope layer 10 is formed. In the field dope layer 10, a part 10a which is in contact with the N-well 6 has high concentration and a part 10b which is in contact with a source/drain 12 an N-type diffusion layer has low concentration. In the P-well 4, an NMOS transistor is formed and a PMOS transistor is formed in a region enclosed by the field oxide film 8. Thereby, junction breakdown strength of the N-type diffusion layer can be improved.