PURPOSE: To attain accurate voice detection with a simple circuit scale and less processing time by utilizing parameters calculated from a vocoder having an acoustic sense weighting filter adaptor and a backward gain adaptor efficiently.

CONSTITUTION: A reflection coefficient (t) calculated by an acoustic sense weighting filter adaptor 106 of a vocoder 100 and an exciting gain (s) calculated by a backward gain adaptor 110 are inputted to a voice detector 200. The voice detector 2000 uses mean value calculation devices 201, 204 to calculate a mean value of an input signal for a predetermined period and a decision circuit 205 compares the mean value with a preset threshold level to make decision on sound/silence. Thus, accurate decision is made under a usual service environment by taking the presence of background noise in account. The quantity required for decision processing is less and the voice is detected for a period less than a 10ms frame unit and the circuit scale is made small.