To provide a technology to reduce not only a driving loss but also a switching loss.

In a power supply unit wherein a high-side and a low-side switching elements are each composed of a plurality of power MOSFETs connected in parallel, the plurality of power MOSFETs connected in parallel are controlled in such a manner that the smaller the output current, the more power MOSFETs are in an off state. The power MOSFETs that turn off when the output current is small are disposed outside a loop from a positive terminal of an input capacitor of a printed board to a negative terminal of the input capacitor through the switching elements. Due to this structure, by turning off packages 141C, 142C of the power MOSFETs disposed outside a main circuit loop and by turning on packages 141B, 142B of the power MOSFETs disposed inside the main circuit loop, a parasitic inductance of the main circuit is reduced, which leads to the reduction of a switching loss and the improvement of the efficiency of a light load.