PURPOSE: To make the respective emitter current values of the power transistors to the same level by a method wherein a conductor itself to mount the power transistor group is utilized as the bus-bar on the emitter side.

CONSTITUTION: The collector terminals C1WC3 of the respective power transistors TR's T1WT3 are connected through the bus-bar P. While, the respective emitter terminals E1WE3 and a heat sink 1 are directly connected electrically through metal fittings 41W43. Moreover the respective TR's T1WT3 are equipped to the heat sink 1 through adapter plates 51W53, and a terminal N1 to lead out load current in the lump is provided to the heat sink 1. According to said construction, the transient characteristics of the TR's at the point in time of turn- ON are equalized respectively, the specified TR only is not subjected to thermal destruction due to the increase of the allotted current. Moreover, because the inductance value of the heat sink 1 itself is smaller as compared with the inductance value of the bus-bar, influences to be applied to the TR's at the point in time of turn-ON can be mitigated.