An auxiliary power supply device includes an auxiliary power supply; and an auxiliary power supply control unit configured to switch a mode of the auxiliary power supply to an output mode or a non-output mode when electric power is output from a main power supply to a drive motor. The auxiliary power supply control unit is configured to switch the mode to the non-output mode when a required power value required by the drive motor is equal to or smaller than a predetermined power value and to switch the mode to the output mode when the required power value is larger than the predetermined power value, the predetermined power value being set to a value smaller than outputtable electric power that is electric power outputtable from the main power supply.