An auxiliary power supply device includes: a resonance-type inverter circuit to convert DC power input from a DC power supply to AC power, a primary coil for input of AC power from the inverter circuit, a transformer for output of AC power from a secondary coil insulated from the primary coil, a converter circuit for conversion of AC power from the transformer to DC power, a filter condenser for smoothing of DC voltage from the converter circuit, and an inverter controller for output of a gate signal for causing operation of switching elements of the inverter circuit. The inverter controller, in a charging mode for charging the filter condenser, makes pulse width of the gate signal smaller than when in a running mode for running of electric rolling stock, and gradually increases the pulse width in accordance with an elapsed time under control in the charging mode.