PURPOSE: To attain the transmission of an acoustic signal with high quality by high efficiency-encoding an acoustic signal by obtaining a number of changes in a primary moment of amplitude components for each discrete frequency and comparing the number with a predetermined value when a digital acoustic signal is subjected to Fourier transformation.

CONSTITUTION: A frame signal segmented from an acoustic signal is subjected to discrete Fourier transformation by using the same window function and the primary moment of the amplitude component for each discrete frequency. Then a frequency Δfi outputted from a subtractor 47 of a primary moment frequency calculation section 45 is compared with a predetermined value at a discrimination circuit 48 based on an adaptive quantization control signal at a bit number information generating part 37 and the result of discrimination is given to a signal generating circuit 49. When the change in the primary moment frequency of the amplitude component of the acoustic signal is large, original data are sent. On the other hand, when the change in the primary moment frequency is small, data with less bit number allocated are formed and the acoustic signal is sent with a high efficiency.