PURPOSE: To prevent the generation of accidents regarding a short circuit resulting due to the adhesion of trash, dust, etc. contained in cooling air by passing cooling air blasted by a blower in hollow hole in a radiator and cooling a thyristor.

CONSTITUTION: An assembly in which thyristors S1WS6 are assembled among radiators F1 and F2, F3 and F4 and F5 and F6 with hollow holes by a pair of fixing fittings 6a, 6b is received in a case 1. Cooling air fed by a blower 13 is diffused in box body 11, passes through the hollow holes of the radiators F to cool the thyristors S, and is discharged from exhuast holes 14. Consequently, since cooling air does not collide directly with thyristor S themselves, trash, dust, etc. contained in cooling air do not adhere on contact section among the thyristors S and the radiators F or the outer surfaces of the thyristors S. Accordingly, there is no possibility of the generation of accidents regarding a short circuit resulting from these adhesion.