An auxiliary power supply device of embodiments includes an electricity storage, a diode, a current sensor, and a controller. The electricity storage is configured to store electric power supplied from a power source supplying the electric power to a load, the electricity storage being connected in parallel to the power source with respect to the load. The diode is configured to allow a current in one direction, the diode being placed in series with the electricity storage. The current sensor is configured to detect the current flowing through the diode. The controller is configured to perform a predetermined control when the current is detected by the current sensor.