PURPOSE: To enable a control unit to be exactly changed over, by setting the ignition pulse signal of the control unit during waiting, to be of phase for setting the main circuit current of a power converter to be zero.

CONSTITUTION: When by a control unit change-over circuit 43, a control unit 41 is selected and controlled, then from a 'waiting' side control unit 42, with phase for enabling main circuit current to be zero, the output of ignition pulse signal c2 is generated. In this case, when a system fault is generated, then by the control unit 41, 'abnormal state' is informed to the control unit change-over circuit 43. Then, immediately by the control unit change-over circuit 43, signal a1 is turned into 'waiting', and signal a2 is turned into 'selection', and the ignition pulse signal c2 of output generated from the control unit 42 as the ignition pulse signal is selected. Besides, after a specified confirmation time limit is set, control is started according to operation sequence.