PURPOSE: To contrive the smallness in size and the lightness in weight of a device total unit improving cooling efficiency by mounting a heat generating unit of resistance or the like to the inside of each cooling fin, in the case of a device housing two or more semiconductor stacks, provided with a semiconductor element and a cooling fin, in a control box mounted under the floor of a vehicle.

CONSTITUTION: A control box 1 stores in its closed chambers 1a, 1b a semiconductor stack 2 and the other electrical parts 3 and arranges in the opening part 1c a cooling fin 5 of the semiconductor stack 2. The opening part 1c is coated with a perforated protective cover 10. A mounting part of a semiconductor element 4 of the cooling fin 5 in the semiconductor stack 2 is constituted of a connection unit 5a for forcing the semiconductor element 4 to come into press contact and a heat radiating unit 5b forming a lattice-shaped ventilating hole 5d. While here in the cooling fin 5, a mounting part, similarly constituted of a connection unit 5a' and a heat radiating unit 5b', for mounting a heating unit 6 of resistance or the like is provided in a position separating a predetermined space in the upper of the mounting part for the semiconductor element 4.