Provided is a technique for preventing warps of cooling plates due to a contraction of a joining material, thereby preventing a reduction in cooling performance of a semiconductor device. The semiconductor device includes the following: a first cooling plate; a second cooling plate facing the first cooling plate; a semiconductor chip joined between the circuit pattern of the first cooling plate and the circuit pattern of the second cooling plate with a joining material; and a case containing part of the first cooling plate, part of the second cooling plate, and the semiconductor chip. The semiconductor chip is mounted in a semiconductor-chip mounting part between the first cooling plate and the second cooling plate. The case is provided with a portion corresponding to the semiconductor-chip mounting part and to surroundings thereof. The portion has an up-and-down width greater than an up-and-down width of the remaining portions of the case.