PURPOSE: To lighten it and improve cooling efficiency by attaching semiconductor stacks, which are constituted by arranging semiconductor elements horizontally each in a line on element mounting bases, to the facing two sides at both end faces of a cooling fin.

CONSTITUTION: Element mounting bases (SB) 3 are provided at the facing two sides of a cooling fin (CF) 2, and semiconductor elements (IC) 1 are attached horizontally each in line on both sides of CF2, that is, on SBs 3 so as to constitute semiconductor stacks 4. Semiconductor cooler mounting plates (SC) 9 and 10 are provided at the tops and bottoms of SBs 3, and SCs 10 are made larger than SCs 9. Moreover, a ventilation groove is made at CF2 to make a heat radiation part 6. Two sheets of mounting frames 11 and 12 are provided above and below in front and in rear inside a controller box 5, and the mounting frame 11 is made smaller than the mounting frame 12, and SC 9 and 10 are attached to them through packings 13. An air duct 14 is made between the mounting frames 11 and 12, and a sealed chamber 15 is made in the space from the mounting frame 12 to the inmost bottom. That is, the heat generated in the IC1 is transmitted to CF2, and is radiated in the heat radiation part 6 to cool IC1.