A semiconductor device including an insulating circuit board. The insulating circuit board has an insulating plate, a plurality of circuit patterns disposed on a front surface of the insulating plate, any adjacent two of the circuit patterns having a gap therebetween, each circuit pattern having at least one corner, each corner being in a corner area that covers the corner and a portion of each gap adjacent to the corner, and a buffer material containing resin, applied at a plurality of corner areas, to fill the gaps in the plurality of corner areas.