Based on ADP-CELP (Adaptive Density Pulse Code Excited Linear Prediction), in which the sound source is expressed in terms of an adaptive density pulse train model, a variable-rate ADP-CELP system is proposed in which one mode out of four, with different bit rate depending on the nature of the short section speech signal, is selected for each frame. The coding method in each mode is designed to be adapted to the nonspeech section, nonsound section, steady speech section, and nonsteady speech section. The bit rates are 0.53, 2.67, 3.17, and 4.1 kbps. In the variable-rate ADP-CELP system, the variation of the spectral envelope is used as a guideline so that a high sound quality and low bit rate are attempted by means of the speech/nonspeech classification method to accurately distinguish the speech section and nonspeech section even under a background noise environment and of the interpolation vector quantization method that can reduce the coding volume for the pitch period to one-half without degrading the quality. In the variable-rate ADP-CELP system, the average rate is less than 2.4 kbps in the case of sound rate of 60% and the corresponding quality is more than the half-rate standard system (3.45 kbps) of domestic digital cellular telephones.