PURPOSE: To prevent the overshoot of an error amplifier under any operating condition.

CONSTITUTION: An output voltage Vout from a DC/DC converter is detected by an output-voltage detection circuit 8. An overshoot prevention circuit 22 is constituted of an auxiliary power-supply circuit 23 and of a current-limiting resistance 24, and its output end is connected to one end of the output-voltage detection circuit 8. An error amplifier 10 compares the detection signal of the output-voltage detection circuit 8 with a reference voltage Vref. On the basis of a compared result, a pulse continuity width from a control circuit 14 is controlled. Immediately before the output voltage Vout rises, a bias voltage Vb is applied to the noninverting input terminal of the error amplifier 10. Thereby, it is possible to prevent the overshoot of the error amplifier 10.