While a digital input is oversampled up to eight times to process and the oversample data into a specified digital fundamental waveform with multipliers/adders (4-10) to carry out only folding operation with delay circuits 11-1-11-4 and the multipliers/adders (12-15) to allow the determination of a continuous interpolating value. Thus, it is sufficient to provide no low-pass filter which causes a deterioration in phase characteristics. A limited number of determined interpolating functions are determined to prevent a truncation error in interpolation. An AND gate (2) is used to determine a part of the oversample data as the input data, so that processing and folding operation of digital fundamental waveform is carried out in a very simple processing.