A DC--DC converter, which can respond quickly and suitably to changes in input voltage within the scope of normal operating conditions to return to the normal operating state. The DC--DC converter is comprised of switching power supply unit 10, which can turn on/off switching elements that can be turned on/off at high frequency to convert a DC input voltage V.sub.in into a DC output voltage V.sub.out, and a control unit 12, which is used to control the on/off operation of the switching elements of said switching power supply unit 10. In control unit 12, when DC output voltage V.sub.out is out of the range of monitoring value AM, the response characteristic of the feedback loop (especially the response characteristic of error amplifier 14) is switched to a greater responsivity to continue the switching control operation of the control system without stopping it.