To provide a digital signal processing apparatus capable of appropriately suppressing generation of noises at switching whether filter processing is required with a digital filter, and a sound reproducing apparatus provided with this processing apparatus.

A signal whose sound quality is processed in a sound quality processing section 10 octuples its sampling frequency in an interpolation filter 20, and the signal is supplied to a D/A converter consisting of a noise shaper 30, a pulse width amplifier 40 and a low-pass filter 50. The signal is converted into an analog signal in this section and is outputted from an output section 60, while a digital signal processing mode by the section 10 is performed, by controlling the coefficients of multipliers 11-15 in a filter control section 80 on the basis of an instruction of a sound quality operation section 70. Particularly, if digital processing is not conducted, the coefficients are modified so that the denominator of a transfer function becomes equal to the numerator thereof.