PURPOSE: To largely improve heat sink effect of an electric part required for heat sink by holding the part on a printed circuit board, soldering together a terminal and directly mounting a body of the part to be required for heat sink on a sheathed case.

CONSTITUTION: A terminal of a collector of a transistor 1 of an electric part required for heat sink is inserted into a wiring hole of a printed circuit board 4, and a holder 7 of a transistor body is formed in a scribing line 6 such as a slit or a hole in the vicinity of the wiring hole to be soldered. In case of the step of soldering, the terminals of other electric parts 5,... are inserted into the prescribed wiring hole of the board 4, the terminals of the collector and emitter of the transistor 1 are inserted into the wiring hole, and the body is held by the holder 7 via holding means 8. When the terminals are soldered together with other electric parts, the holder 7 is isolated from the substrate 1 at the scribing line 6, the means 8 is removed from the mounting hole 1' of the body, the substrate is fixed to a sheathed case 2, and the body of the transistor is secured through the hole 1' and the case by screws or the like.