A semiconductor module mounted on a vehicle is disclosed. The semiconductor module includes a semiconductor element, a forced-cooling type cooler, and a heat mass. Heat generated in the semiconductor element is conducted to the cooler. The heat mass is joined onto the semiconductor element, so as to be thermally coupled to the semiconductor element. The heat mass is formed such that the thermal resistance of a part of the heat mass that corresponds to a high temperature part of the semiconductor element in a heat generating state is lower than the thermal resistance of a part of the heat mass that corresponds to a lower temperature part of the semiconductor element.