Abstract: In an APC system waveform dam compression apparatus which generates an optimum prediction coefficient utilizing a block calculation process in a first mode and generating prediction data utilizing a block calculation process based on this optimum prediction coefficient in a second mode with respect to a fixed number of blocked waveform data, to make possible use of suitable data in the first prediction calculation process of each block calculation process. A data holding section for holding data corresponding to at least final first waveform data to be used in a final prediction calculation process of a block calculation process in a second mode, and a first selection section for using data held in the data holding section in place of data memorized in a second delay section in each first prediction calculation process of each block calculation process of the next block are provided in an operating circuit.