PURPOSE: To solve the problems of a conventional device which has a larger waveform distortion and an increased quantization error.

CONSTITUTION: The device is provided with a band dividing filter 1 which divides digital signals of voice signals into plural bands, an electric power computing means 2 which obtains the power of the output of the filter 1 for every block and quantizes them in a form of 2σ power, a quantizing means 6 which adaptively quantizes for every block and a quantized bit number control section 5 which adaptivley controls quantized bit numbers of each band of the means 6 based on the value of σ. The section 5 regularly varies the quantization bit number for every sample data outputted from the filter 1 and sets the quantization bit number of the means 6, that is assigned to each band, to a number which is larger than zero including decimals. It is desired that the section 5 sets a bit number which is larger than zero and less than 1 for the band that has a bit number assigned to the means 6 is less than zero.