To decrease heat generation increase related to a diode in a failure in a power device, by causing a power circuit having failed to send a failure information signal to another normal power circuit, and causing the normal power circuit to by-pass the diode on the basis of the failure information signal and to output a defined source voltage.

Power circuits 1, 2 are constituted redundantly in parallel through the output ends of diodes 12, 22 inserted into the output lines 11, 21 of power generating sections 10, 20 for generating a specified power voltage. The power circuits 1, 2 are provided with first relays 14, 24 for performing switching to by-pass individual diodes 12, 22, and second relays 15, 25 for detecting the failure of the power sources. The contact of the relay is connected with a relay 15c if a given power is supplied from the power circuit 1, and the contact of the relay is connected with a relay 15b if the power supply is interrupted. To the common terminal 25a of the second relay 25 in the power circuit 2 a source voltage +5 V is supplied from a power generating section 10, and the second relay 25 is driven by the power output of the power circuit 2.