PURPOSE: To improve the bit compression rate without deteriorating sound quality by applying inverting processing after a difference data subject to bit compression in the unit of blocks is subject to bit expansion.

CONSTITUTION: Control information DCO and amplitude information DAM read from a memory are fed to a bit expansion circuit 22 via switches 23, 24, an output from the bit expansion circuit 22 is fed to a local decoder 30, where the processing being inverse to the processing forming a difference data and a reproduced sound data D'AU is outputted from a data output terminal 12. After the difference data (amplitude information DAM) subject to bit compression by the bit compression circuit 21 is subject to bit expansion by the bit expansion circuit 22, the inverse processing to the processing formint the difference data is applied by the local decoder 30, the recovered data DRE obtained by it is fed to a difference processing circuit 6 to form a difference data DDI fed to the bit compression circuit 21. Thus, the error caused at bit compression/ expansion is adsorbed.