A high breakdown voltage semiconductor device is formed using an SOI substrate comprising a support substrate, an insulating film, and an active layer. The high breakdown voltage semiconductor device comprises an N-type well region and a P-type drain offset region formed on the active layer, a P-type source region formed on the well region, a P-type drain region formed on the drain offset region, a gate insulating film formed in at least a region interposed between the source region and the drain offset region of the active layer, and a gate electrode formed on the gate insulating film. The device further comprises an N-type deep well region formed under the drain offset region. A concentration peak of N-type impurity for formation of the deep well region is located deeper than a concentration peak of P-type impurity for formation of the drain offset region.