PURPOSE: To prevent the deterioration in sound quality in the audible sense even with an increased compression rate by applying the level correction in response to an input amplitude level in the unit of blocks according to a charac teristic inversed to the Fletcher-Munson equi-loudness curves, coding the input and sending it, restoring the amplitude level by a characteristic inverse to the former characteristic so as to listen to the sound after decoding.

CONSTITUTION: An input signal being an electric signal converted and amplified from an audible sound signal is inputted to a selection amplifier circuit 2, where the signal is amplified or compressed. Then an output of each selection amplifier circuit is stored tentatively in a buffer memory circuit 3. Peak hold circuits 4-1∼4-4 hold the peak value of the output of each selection amplifier circuit. A maximum position detection circuit 5 reads a value held in the peak hold circuit 4 at each prescribed time, selects one of output data within a prescribed time of the selection amplifier circuit, and a signal coding circuit 7 applies quantization and coding. The coded data is sent to a signal decoding circuit 9 and a selection decoding circuit 10, where the data is decoded. The decoded input signal is inputted to an inverse selection amplifier circuit 11, one of the outputs is selected and given to an output terminal 12.