A method determines tap coefficients of a phase interpolation finite impulse response (FIR) filter with respect to a digital-signal phase-locked loop processing apparatus including an A/D converter receiving a reproduced signal serving as digital data and sampling the received reproduced signal at a frequency higher than a data rate frequency and a PLL portion performing phase locking of the reproduced signal by filtering the sampled reproduced signal obtained by the A/D converter using the phase interpolation FIR filter and outputting a data sequence of the phase-locked reproduced signal. The method includes providing an adaptive equalization FIR filter at the data rate frequency at a stage subsequent to the PLL portion and determining the tap coefficients of the phase interpolation FIR filter using, as estimated values, FIR filter tap coefficients of the adaptive equalization FIR filter, which are converged so as to achieve a minimum mean square error.