In a DC-DC converter using, as an input source, a fuel cell, a solar cell, or the like, having a relatively large output impedance in a power-supply mode, an arbitrary load and secondary battery based on magnetic coupling in which voltage values are determined by a turns ratio between those wound around a winding, are provided as outputs, a second converter is connected between an output of the secondary battery and the load. In addition, a current control circuit connected to the secondary battery is configured to perform control so that, when a current in the load decreases due to an operation of the magnetic coupling, by increasing a charging current which flows into the secondary battery, and, when the current in the load increases, by decreasing the charging current to the secondary battery, an output voltage of the secondary battery is maintained at a set drooping voltage.