PURPOSE: To modulate/demodulate a faithful original waveform without increasing a sampling frequency by adding an integrated value at every one-sampling period of the difference signal between a signal before modulation and a delay signal to a delay signal.

CONSTITUTION: Quantized data after PCM modulation is converted by an amplitude discrete/continuous converter 1 into a rectangular wave signal having an amplitude corresponding to each quantized data. Its output A is delayed by the delay circuit 2 by one sampling period and outputted as a signal B. The signals A and B are inputted to a subtracter 3 to generate the difference signal C(=A-B). The signal C is integrated by a reset integration circuit 4 at every sampling period. The output of the circuit 4 and the output signal B are added by an adder 5 to generate a continuous signal E. The signal E is a signal generated by linear interpolation between sample points in a discrete state and contains no sampling frequency component, so a side band due to the sampling frequency and the modulation of the original signal is not generated.