PURPOSE: To conduct self-cooling effectively by arranging upper stage, lower stage semiconductor elements and a cooling fin in a displaced manner with each other.

CONSTITUTION: The semiconductor elements 3 and the cooling fins 4 disposed in an opining 1 of a casing are housed in the casing 2. An inside wall 5 has stage sections in the vertical direction, and the fins 4 on the backs of the elements are shifted in the horizontal direction when the elements 3 are attached on the wall 5. The casing is mounted at a clearance G apart from a car body 8, and a rising hot air passes smoothly through a gauze 9. According to this constitutuion, maintenance and inspection are conducted conveniently, and the outside air besides the hot air at the lower stage contact directly with the fin at the upper stage. Consequently, overall cooling effect is improved.