A self-excited multi-phase DC-DC converter having satisfactory responsiveness when its load suddenly changes. A control unit of the converter compares output currents of first and second converter units. Based on the comparison result, the control unit generates control signals to operate a converter unit through which a smaller output current flows. For example, when an output voltage of the converter decreases due to a sudden change in the load while the first converter unit is operating to supply current, the second converter unit through which a smaller output current flows is operated to increase the output voltage.