To provide a step-down type DC-DC converter which maintains high efficiency even when a load current is small, and to provide a variable smoothing inductor for use in the same.

The reducing type DC-DC converter is constituted by employing a smoothing inductor with a variable inductance which is constituted of coils 1-5 electrically connected in series to electrode terminals 6-11 and provided on an insulating substrate 12, while both ends of series connecting body composed of electric connecting units for the series connection and all coils are electrically connected to the electrode terminals 6-11, respectively. In this case, the coils 1-5 can use cores composed of a soft magnetic material. Further, coils 13-17 can have a core 18 in common.