Water paths for feeding a coolant water through a power converter mounted on an automobile are arranged in parallel, openings are formed on the water paths respectively, heat radiating fins project from the openings, and the openings are closed by a base plate of the power module. Further, the base plate of the power module includes a metal in addition to copper to increase a hardness of the base plate, so that a deterioration of the flatness during fixing the fins with brazing is restrained.