A resin-sealed semiconductor device, including a lead frame. A semiconductor chip is formed on the lead frame and is electrically connected with the lead frame. At least a part of the lead frame and the semiconductor chip are sealed with a theremosetting resin which includes a base resin, aluminum oxide and fused silica. Stress acting on the semiconductor chip is reduced because the thermal expansion coefficient of the thermosetting resin becomes low due to the fused silica. At the same time the thermal conductivity of the thermosetting resin is increased because of the aluminum oxide.