A signal corresponding to a short-period change and a signal corresponding to a long-period change of a sound signal are detected, and optimal quantization is performed based on the combination, of the two signals. In an ADPCM encoding apparatus (100), a differential value d n between a 16-bit input signal X n and a decoded signal Y n-1 of one sample ago is calculated by a subtractor (102). Thereafter, the 16-bit differential value d n is adaptively quantized by an adaptive quantizing section (103), so as to be converted to a (1 to 8)-bit length-variable ADPCM value D n . Thereafter, the ADPCM value D n is compression-encoded by a compression-encoding section (108) to generate a signal D' n , and the signal D' n is framed by a framing section (130) and outputted. Further, in an ADPCM decoding apparatus, a framed input signal is subjected to a reverse of the aforesaid process so as to be decoded.
