A high breakdown voltage semiconductor device. The device includes a semiconductor substrate, an insulating film formed on the semiconductor substrate, an active region formed on the insulating film, drain and base regions formed in a surface portion of the active region, and a source region formed in a surface portion of the base region. First and second gate insulating films are formed on inner surfaces of first and second grooves penetrating the base region so as to come in contact with the source region and reaching the active region, with first and second electrodes being buried in the first and second grooves. Two or more channel regions are formed in a MOS structure constructed by the gate insulating film, the gate electrode, the source region, the base region and the active region.