The present invention includes a holding stay made of a heat conductive material that is the same as that of an isolator holder, the holding stay being in contact with a radiation stay made of a member having good thermal conductivity, the radiation stay being in contact with radiation fins extracted from the inside of the isolator holder through an external opening for extraction, columnar welded portions bond the holding stay and the isolator holder through openings for welding, the welded portions apply tensile force toward the isolator holder to the radiation stay via the holding stay, and the radiation stay presses the radiation fins by means of the above-described tensile force to be fixed to the isolator holder.