The invention provides a high voltage MOS transistor having a high gate breakdown voltage and a high source/drain breakdown voltage and having a low on-resistance. A gate electrode is formed on an epitaxial silicon layer with a LOCOS film being interposed therebetween. A P-type first drift layer is formed on the left side of the LOCOS film, and a P+-type source layer is disposed on the surface of the epitaxial silicon layer on the right side of the LOCOS film, being opposed to the first drift layer over the gate electrode. A P-type second drift layer is formed by being diffused in the epitaxial silicon layer deeper than the first drift layer, extending from under the first drift layer to under the left side of the LOCOS film. A recess is formed in a bottom portion of the second drift layer under the left end of the LOCOS film.