A vehicle-mounted power supply apparatus (1, 10), comprising: a first battery (2, 11); a second battery (3, 12) configured to output an activation voltage lower than an output voltage of the first battery (2, 11); an electrical junction box (4, 13) including a plurality of relays and a controller, the plurality of relays configured to be connected or disconnected using the activation voltage to distribute and output electric power received from the first battery (2, 11) or shut off the electric power received from the first battery (2, 11), the controller configured to control the connection or the disconnection of the plurality of relays using the activation voltage to cause the plurality of relays to be connected upon activation and to be disconnected upon stopping of an operation; and a backup power supply including a power storage, a contactless power receiver, and a discharger, the backup power supply configured to supply driving power to a battery relay that is one of the plurality of relays, the backup power supply configured to supply driving power to the controller, the contactless power receiver configured to be connected to a charging path of the power storage and to contactlessly receive electric power, the discharger configured to be connected to a discharging path of the power storage, the battery relay configured to supply charging power from the first battery (2, 11) to the second battery (3, 12), wherein as the activation voltage output from the second battery (3, 12) is lower than a lower limit value that is predetermined, the power storage is configured to be charged with electric power supplied from outside of the vehicle-mounted power supply apparatus to the contactless power receiver, the electric power stored in the power storage is configured to be supplied to the controller via the discharger to enable the controller to be activated, after the controller is activated, the controller is configured to cause the battery relay to be connected using the electric power supplied from the power storage via the discharger, and electric power is supplied from the first battery (2, 11) to the second battery (3, 12) to enable the second battery (3, 12) to output a voltage higher than or equal to the lower limit value and enable the plurality of relays to be connected. a first battery (2, 11); a second battery (3, 12) configured to output an activation voltage lower than an output voltage of the first battery (2, 11); an electrical junction box (4, 13) including a plurality of relays and a controller, the plurality of relays configured to be connected or disconnected using the activation voltage to distribute and output electric power received from the first battery (2, 11) or shut off the electric power received from the first battery (2, 11), the controller configured to control the connection or the disconnection of the plurality of relays using the activation voltage to cause the plurality of relays to be connected upon activation and to be disconnected upon stopping of an operation; and the plurality of relays configured to be connected or disconnected using the activation voltage to distribute and output electric power received from the first battery (2, 11) or shut off the electric power received from the first battery (2, 11), the controller configured to control the connection or the disconnection of the plurality of relays using the activation voltage to cause the plurality of relays to be connected upon activation and to be disconnected upon stopping of an operation; and a backup power supply including a power storage, a contactless power receiver, and a discharger, the backup power supply configured to supply driving power to a battery relay that is one of the plurality of relays, the backup power supply configured to supply driving power to the controller, the contactless power receiver configured to be connected to a charging path of the power storage and to contactlessly receive electric power, the discharger configured to be connected to a discharging path of the power storage, the battery relay configured to supply charging power from the first battery (2, 11) to the second battery (3, 12), wherein the contactless power receiver configured to be connected to a charging path of the power storage and to contactlessly receive electric power, the discharger configured to be connected to a discharging path of the power storage, the battery relay configured to supply charging power from the first battery (2, 11) to the second battery (3, 12), wherein as the activation voltage output from the second battery (3, 12) is lower than a lower limit value that is predetermined, the power storage is configured to be charged with electric power supplied from outside of the vehicle-mounted power supply apparatus to the contactless power receiver, the electric power stored in the power storage is configured to be supplied to the controller via the discharger to enable the controller to be activated, after the controller is activated, the controller is configured to cause the battery relay to be connected using the electric power supplied from the power storage via the discharger, and electric power is supplied from the first battery (2, 11) to the second battery (3, 12) to enable the second battery (3, 12) to output a voltage higher than or equal to the lower limit value and enable the plurality of relays to be connected.