A semiconductor device is disclosed that comprises a high breakdown voltage MOSFET. The MOSFET includes a source region of a second conductivity type and a drain region of the second conductivity type formed apart from each other in a well region of a first conductivity type, a channel region formed between the source region and the drain region, a gate insulation film formed on the channel region, a LOCOS oxide film having a greater film thickness than the gate insulation film and formed adjacent to the gate insulation film, and a gate electrode formed across the gate insulation film and the LOCOS oxide film.