The present invention provides a waveform data compression apparatus which can express waveform data by a small amount of compression data obtained by compressing the aforesaid waveform data, and a waveform data expansion apparatus which can reproduce the compression data produced by the aforesaid waveform data compression apparatus to obtain the original waveform data. The. waveform data compression apparatus interpolates waveform data of addresses before and behind the waveform data of an objective address in a block to be processed to acquire interpolated data of the objective address to be processed, and uses difference data between the interpolated data and the waveform data of the objective address to be processed, thereby compressing the waveform data. The waveform data expansion apparatus operates to perform reverse process to that performed by the waveform data compression apparatus.