A semiconductor device is disclosed that includes an insulation substrate, a metal wiring layer, a semiconductor element, a heat sink, and a stress relaxation member located between the insulation substrate and the heat sink. The heat sink has a plurality of partitioning walls that extend in one direction and are arranged at intervals. The stress relaxation member includes a stress absorbing portion formed by through holes extending through the entire thickness of the stress relaxation member. Each hole is formed such that its dimension along the longitudinal direction of the partitioning walls is greater than its dimension along the arranging direction of the partitioning walls.