An auxiliary power source device for a vehicle is incorporated in an electric vehicle and includes a three-phase inverter that converts an input DC voltage into a desired three-phase AC voltage and applies the three-phase AC voltage to a load. The auxiliary power source device further includes a filter reactor that is connected to respective output terminals of a three-phase inverter, a filter capacitor that is connected in a Y-shape at an end on a load side of the filter reactor and is not grounded at a neutral point, and a three-phase transformer that includes primary windings that are connected in a Y-shape at the end on the load side of the filter reactor and is grounded at a neutral point and secondary windings that are connected in a delta shape.