To provide a DC-voltage converting circuit, capable of reducing response delays in a feedback loop and stabilizing output voltage.

This converting circuit includes a differential voltage generating means (22) for generating differential voltage between detected voltage of the output voltage, in which DC-voltage supplied by a DC power source is stepped vertically and a reference voltage, a comparing means (28) for comparing the differential voltage with a ramp waveform signal and generating a switching pulse of a duty ratio which corresponds to the differential voltage, and a slow start means (30) for generating a slow start signal of fall waveform voltage which falls gradually from prescribed voltage when power source is turned on, supplying the signal to the comparison means 28 instead of the differential voltage, and increasing the duty ratio of the switching pulse gradually. Thus the response delay in the feedback loop can be reduced and stabilize the output voltage.