In a digital control power supply, a mode control unit measures a first frequency and a second frequency for a difference between a second digital value and a target value. Based on the measured first frequency and second frequency and a predetermined threshold set to the first and second frequencies, the mode control unit determines whether an amplification factor for use in amplification processing by an amplifier is maintained at a current amplification factor or is changed to an amplification factor which is larger or smaller by 1 than the current amplification factor. This contributes to an improvement in noise resistance of the digital control power supply and prevents an output voltage from being unstable.