A digital .DELTA..SIGMA. modulated signal produced by a quantizer (1c) in a .DELTA..SIGMA. modulation processing unit (1) is put in to a feedback loop before inputted to a power switch (2) and converted into an analog signal by a D/A converter (1d). This analog signal is fed back to the input stage (a differentiator (1a)) of the .DELTA..SIGMA. processing unit (1) to prevent the switching distortion of the power switch (2) itself from being introduced into the feedback loop thereby to suppress the influence of the switching distortion.