A vehicle auxiliary electric-power-supplying system can normally stop an electric power inverter by the frequency in use for an electric power supplier being suppressed as low as possible, and electric power being immediately started to be supplied from the power supplier to a controller in a case in which normal electric power has become unable to be obtained from power-outputting of the electric power inverter. The system includes: the electric power inverter for converting a first type of dc power received through an overhead wire to a second type of dc power, and supplying the second type of dc power to a dc load; the power supplier for converting the first type of dc power received through the overhead wire to a third type of dc power; a power-outputting unit, connected to both the electric power inverter and the electric power supplier, for outputting either the second type of dc power or the third type of dc power; and the controller for receiving power from the power-outputting unit, and controlling the electric power inverter.