A vehicle auxiliary power supply that is mounted on a railway vehicle, includes a three-phase inverter circuit that converts DC power or AC power input from an overhead line to desired AC power to supply the AC power to a load, and is connected in parallel with a VVVF inverter device that drives a propulsion motor, wherein a blocking diode that prevents backflow from a side of the three-phase inverter circuit to a side of the overhead line is provided between the overhead line and the three-phase inverter circuit, and a SiC Schottky barrier diode is applied to the blocking diode.