In a DC-DC converter of synchronous rectifying system, a control circuit of DC-DC converter capable of improving the control voltage value of output voltage at light load and enhancing the power conversion efficiency, and its control method are presented. An inverting input terminal of comparator COMP2 of which non-inverting input terminal is connected to a grounding potential is connected to one terminal of choke coil L1 by way of terminal (X). An output terminal (N1) of comparator COMP2 is connected to a delay circuit DL, and an output terminal (N2) of delay circuit DL is connected to AND gate circuit AND1. The delay circuit DL is a circuit for delaying the transition from high level to low level of input terminal (N1). Excessive power supplied more than demanded by the load at light load is returned to choke coil L1. Elevation of output voltage VOUT due to excess power accumulated in output capacitor C1 can be suppressed.