A p well serving as a channel region of a MOSFET is formed on one side of an n.sup.- layer and an n.sup.+ drain region is formed on the other side. Above the n.sup.- layer, a plurality of first floating field plates are formed with a first insulating film interposed therebetween. A plurality of second floating field plates are formed thereon with a second insulating film interposed therebetween. Assuming that the thickness of the first insulating film is "a" and the distance between the first floating field plates and the second floating field plates in a direction of thickness of the second insulating film is "b", a relation a>b is held.
