A semiconductor device includes: a wiring board including an insulating board and a wiring layer, the insulating board having an element mounting surface, which is a first main surface, and a back surface, which is a second main surface on the opposite side of the element mounting surface, the wiring layer being formed on the back surface and including a wiring portion and a heat dissipation portion; a power element that is a semiconductor element, is mounted on the element mounting surface of the wiring board, and is connected to the wiring portion; a spacer that is interposed between the power element and the element mounting surface of the wiring board and is connected to the back-surface-side heat dissipation portion; and a heatsink that sandwiches, together with the spacer, the power element and is secured to the spacer.