A semiconductor device includes a base plate, a semiconductor chip, a case, a heat dissipation member, and a plurality of attachment portions. The semiconductor chip is held on a front surface side of the base plate. The case is provided on a front surface of the base plate so as to house the semiconductor chip inside. The heat dissipation member is provided on a back surface of the base plate and contactable with a heat sink for cooling the semiconductor chip. The plurality of attachment portions have a function of attaching the case to the heat sink. Ends of the heat dissipation member in a direction extending along a long side of a plurality of sides that form a shape defined by connecting positions of the plurality of attachment portions in plan view are located between the two attachment portions that form the long side.