PURPOSE: To reduce the noise of an apparatus with a simple circuit constitution by connecting a third diode between the input side terminal of a resonating inductor and a power source negative electrode.

CONSTITUTION: A switching power source circuit is composed of a switching element SW, a first diode 6, resonating inductor 8 and capacitor 9, a second diode 11, and load side choke coil 12 and capacitor 13 to supply power from output terminals 3, 4 to a load L. In this case, a third diode 10 is provided between the junction and terminals of said resonating inductor 8 and load side choke coil 12, and respective inductor and capacitor are formed so that there exists the relation of L1L2 and C1C2 between them. Thus, at the point of time when the switching element SW is turned OFF and a current source for a circuit current iL1 disappears, said third diode 10 is turned ON and said circuit current decreases gradually so that arc is self-extinguished. As a result, it is possible to prevent generation of a surge voltage.