A field-effect transistor is on an insulating surface, and an interlayer insulator film covers the field-effect transistor. An electroconductive field plate is on the interlayer insulator film. The field-effect transistor includes a source region with first electroconductivity, a body region with second electroconductivity, a drift region with the first electroconductivity, a drain region with the first electroconductivity, and a gate electrode. When the insulating surface is viewed in plan, the gate electrode has a shape elongated in a first direction, and the body region is below the gate electrode to overlap the gate electrode. The source region and the drift region are at positions to hold the body region therebetween in a second direction orthogonal to the first direction, and the drain region is at a position farther than the drift region is when viewed from the body region.