To provide a DC-DC converter by which a high efficiency can be obtained both at the time of a light load and at the time of a heavy load.

An input capacitor Ci, a switching device Q1, a diode D1, a smoothing circuit 11, and a voltage-dividing resistance 12 are inserted between the input terminals V1 and V1' and the output terminals V2 and V2' of a DC-DC converter 10. A switching device drive circuit 13 is connected to the gate of an N-MOSFET, which is the switching device. Further, a frequency variable circuit 16, composed of an oscillation circuit 17 and a ripple voltage detection circuit 18 is connected between the output terminal V2 and a pulse width modulation circuit 15.