Abstract: The present invention provides a novel DC-DC converter in which, by achieving constant power control that sets power supply on an output side to be equal to supply power of a fuel cell on an input side, a cell charging current is set to be constant, components are reduced, and stable charging is performed. In a DC-DC converter using, as an input source (Vfc), a fuel cell, a solar cell, or the like, having a relatively large output impedance in a power-supply mode, an arbitrary load (Ro) and secondary battery (B), 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 (30) is connected between an output of the secondary battery and the load.