To provide a semiconductor device which can effectively relax electric field concentration at an end of a gate electrode on a drain side.

A semiconductor device comprises: a first well 3 of a first conductivity type formed on a substrate 1; a second well 4 of the first conductivity type which is formed in the first well and has higher concentration than the first well; a source region 11 of a second conductivity type formed in the second well 4; a first drain region 5 of the second conductivity type formed in the first well 3 at a distance from the second well 4; a second drain region 12 of the second conductivity type which is formed in the first drain region 5 and has higher concentration than the first drain region 5; gate insulation films 6, 7; a gate electrode 8; a dummy electrode 9 formed on the first drain region 5 with the gate insulation film 7 interposed therebetween at a distance from the gate electrode 8 and on the second drain region 12 side of the gate electrode 8; and sidewall insulation films 10. A film thickness of the gate insulation film 7 on the second drain region 12 side is thicker than a film thickness of the gate insulation film 6 on the source region 11 side and thinner than a film thickness of an element isolation region 2.