An output detection feedback section (5) for controlling the power supply output voltage of a converter unit (4) in response to an error signal Ve has an error amplifier (26) for outputting the error signal Ve, a phase compensation circuit (41), a state detection circuit (42), and a response compensation circuit (43) for setting the voltage of a capacitor (29) of the phase compensation circuit (41) to a predetermined value. When output voltage is switched, the output voltage is fixed at the predetermined value in a DC manner while a transitional response of the error signal (Ve) is accelerated, so that the output voltage is changed at a high speed to a desired value and the overshoot and undershoot are suppressed around the desired value.