PURPOSE: To perform the reset of a saturable reactor with simple constitution and small amount of loss by providing a duty factor control means, a switching element, the saturable reactor, and a variable capacitance element on a switching power source circuit.

CONSTITUTION: The switching power source circuit 20 of magnetic amplification system is provided at a transmission circuit. The switching power source circuit 20 is constituted of an input side circuit 22, an output side circuit 23, and a PWM controller 24 which supplies a duty factor signal controlled corresponding to a diagnostic condition to the input side circuit 22. The input side circuit 22 is provided with a full wave rectifier circuit 25 and the switching element Q consisting of a CMOS transistor. Also, the output side circuit 23 is provided with the saturable reactor L1, a diode D5, and a choke coil L2. The variable capacitance element Cr is connected in parallel with the diode D5.