To satisfactorily compress/expand a ΣΔ-modulated digital audio signal.

The ΣΔ-modulated digital audio signal outputted from a comparator 4 is supplied to a register 7(15) and a symbol (n/j, k) is supplied to a ROM 8. Besides, the signal in the register 7 is supplied to a register 9, and a preceding symbol (m/i) is supplied to the ROM 8. A conversion table (dictionary) prepared by respectively allocating codes (x) of short bits to symbols having high generation probability is written in this ROM 8. Then, the code (x) from this ROM 8 is supplied through a register 10 to a selector 11. Further, when these symbols (m, n/i, j, k) are combined as prescribed, at this selector 11, the code from the register 10 is selected but in the other case, the prescribed code (x) is first selected and the signal from the register 7 is selected later. Thus, data are compressed.