To provide a reform technique for a ringing choke converter which can suppress the width of ripple of switching frequency, even if the width of load ripple is extremely large and can improve efficiency as well as improve noise property.

There is a turn-on period control system, which detects the change of the circuit operation at turn off of a switching transistor Q1 and forcibly turns on the Q1 after a given time which is set in advance from the detection period. This control system comprises a transistor Q3 for the control of conductivity different from Q1, a resistor R3, and a capacitor C3. The collector of Q3 is connected to the junction between the resistor R2 and the capacitor C2, and the emitter of the transistor Q3 is connected to a base winding L3, and the resistor R3 is connected between the base of the transistor Q3 and one end of the base winding L3, and the capacitor C3 is connected to the base and the emitter of the transistor Q3.