To suppress surge voltage due to the vibrations of a drain current in a switching power supply device, using an SJ-MOSFET.

The switching power supply device is switched, by switching elements of the MOSFET having a superjunction structure. The switching power supply device includes a diode for damping the vibrations antiparallel-connected to the switching elements. When the characteristic curves of the output capacitances Coss of the switching elements to a voltage VDS between a drain and a source are made to resemble closely by a first straight line, a second one and a third one corresponding to a straight line A, one B and one C in Fig.2, the junction capacitance CD2 of the diode for damping vibrations, corresponding to the intersection (b) of the characteristic curve of the junction capacitance CD2 of the diode for damping vibrations and the second straight line, is 40% or more to the output capacitance Coss of the switching element, corresponding to the intersection (a) of the first straight line and the second straight line.