in that order after repeated temperature change in power cycles in which the power semiconductor chip 2 is energized intermittently the connecting portion 5 a of the dummy 5 a lowest in connection strength is first separated from the electrode 3 fig3 b and the subsequent temperature change separates the dummy wire 5 b of the next lowest connection strength from the electrode 3 fig3 c after further repetition of temperature change in power cycles in which the power semiconductor chip 2 is energized intermittently the main circuit wire 4 also comes to be separated from the electrode 3 in view of the fact that the wires are separated in ascending order of connection strength the service life of the connecting portion of the main circuit wire 4 can be predicted in steps by detecting the separation of the dummy wires 5 a 5 b in addition a plurality of dummy wires may have the same connection strength there are variations in the actual connection strength and the phenomenon of separation and a dummy wire is not necessarily separated at a predicted time of degeneration in spite of a variation in dummy wire separation therefore the separation of the main circuit wire can be positively predicted by detecting the separation of any of the dummy wires the known methods of connecting.