A voltage clamping circuit which operates in a stable manner and a switching power source device which enables high-speed operation. In the switching power source device, one source/drain route is connected to an input terminal to which an input voltage is supplied, a predetermined voltage to be restricted is supplied to a gate, and using a MOSFET which provides a current source between another source/drain route and a ground potential of the circuit, a clamp output voltage which corresponds to the input voltage is obtained from another source/drain route. The switching power source device further includes a first switching element which controls a current in an inductor and a second switching element which clamps a reverse electromotive voltage generated in the inductor. The voltage clamping circuit is used in a feedback route for setting a dead time.