PURPOSE: To miniaturize the device by a method wherein an auxuliary mounting plate supported by a mounting plate is set up in front of the mounting plate, a semiconductor element is disposed to the rear munting plate, an accessory apparatus for protecting the element is arranged to the front auxiliary mounting plate and the auxiliary mounting plate is formed so as to be opened to the one side.

CONSTITUTION: The auxiliary mounting plate 11 is arranged at the front side of the mounting plate 2, to the back thereof a cooling fin 1 is fitted, while being separated from the front side, one side section of the plate 11 is rotatably supported at the end section of a supporting fitting 12A fixed to the mounting plate 2 through a hinge 13, and the other side section is detachably fastened to another supporting fitting 12B fixed to the mounting plate 2 by means of a bolt. A plurality of the flat type semiconductor elements 4 each clamped by means of fastenings 3 and the connecting conductors 5 are disponsed to the front of the mounting plate 2, and a plurality of accessory apparatus 8 for severally protecting the elements 4 are arranged to the front of the auxiliary mounting plate 11.