The invention provides an auxiliary power supply system and an operation method for the same. A purpose of the present invention is to prevent system down when a connection failure and the like occurs in a vehicle connection joint box connected between output lines of the auxiliary power supplies. An auxiliary power supply system, particularly mounted on a plurality of railway vehicles, includes a plurality of auxiliary power supplies 31, 32 including an inverter 4 for converting power received from an overhead line 1 into AC power. Vehicle connection joint boxes 111 to 113 are placed at positions joining vehicles between the output lines 91 to 95 and an electromagnetic contactor 12 is provided between the output lines. The auxiliary power supply includes a control device for controlling the inverter and the electromagnetic contactor. The control device opens the electromagnetic contactor when detecting an abnormal state of an input or output of the inverter equal to or more than a predetermined number of times within a predetermined time.