PURPOSE: To readily inspect and favorably cool by a method wherein, in a lower part of a closing chamber-which receives a semiconductor element and in which an inspection cover is arranged on the side of a car body, an opening chamber is formed in a part on the central side of the car body and a heat radiation part of a cooling device is arranged therein.

CONSTITUTION: In a semiconductor device 15A mounted under a floor of a vehicle, a closing chamber 16A and an opening chamber 17A are provided. The opening chamber 17A is formed in a part near a center of a car body 1 in the lower part of the closing chamber 16A, and a part other than a boundary plane on the closing chamber 16A is formed with a ventilation material such as wire net. Inside this, a heat radiation fin part 9 of a heat pipe type cooling device 8 for cooling a semiconductor element is arranged. On the other hand, the closing chamber 16A is provided with an opening part on the side and an inspection cover 18A capable of opening or closing readily is mounted thereto, and components having high inspection frequency are arranged therein on the side of the inspection cover 18A. Thus, it becomes easy to maintain and a temperature increase of the semiconductor element can be reduced to enhance reliability.