A semiconductor device includes a first insulating circuit board, a semiconductor element on the first insulating circuit board, and an encapsulating body. The first insulating circuit board includes a first insulating substrate, and a first inner conductor layer, and a first outer conductor layer. The first inner conductor layer is electrically connected to a first electrode of the semiconductor element inside of the encapsulating body. The first outer conductor layer is exposed from a surface of the encapsulating body. The first inner conductor layer has a first thin-wall portion a thickness of which reduces toward an outer side, along an outer peripheral edge of the first inner conductor layer with a first width. The first outer conductor layer (i) does not have or (ii) has a second thin-wall portion along the outer peripheral edge of the first outer conductor layer with a second width.