A vehicle auxiliary power supply device includes a resonant inverter circuit that converts DC input into a desired AC voltage and outputs the AC voltage and a control unit. The control unit includes a resonance-time managing unit managing resonance time of current flowing in the resonant inverter circuit, a gate-off-command generating unit detecting overcurrent flowing in the resonant inverter circuit based on detected current of a current detector, and, when the overcurrent is detected, generating, based on detected current of the current detector and resonance time managed by the resonance-time managing unit, a gate-off command to turn off switching elements included in the resonant inverter circuit after the elapse of time after which current flowing in the switching elements becomes zero for the first time, and a gate-signal generating unit generating a gate signal that controls the switching elements to be turned off when the gate-off command is input.