A step-down switching DC-DC converter may include an input coil on the
input side, an output coil on the output side, a switch, an output
capacitor, a first series circuit connected between a connection point on
which the input coil and the switch are connected each other and a
negative pole of a power source, and a second series circuit connected
between a connection point on which the output coil and the switch are
connected to each other and the negative pole of the power source. A
connection point between a first intermediate capacitor and the first
intermediate coil is connected to a connection point between a second
intermediate capacitor and the switch via a switching device cooperating
with the switch. The input coil and the first intermediate coil are
electromagnetically coupled with each other. The output coil and the
second intermediate coil are electromagnetically coupled with each other.