1. A voice activity detecting device comprising: a speech-segment inferring section for determining, for each of active voice frames as an aural signal given in order of time sequence, a probability that the active voice frame belongs to an active voice segment, the determining being made based on a statistical characteristic of the aural signal; a quality monitoring section for monitoring quality of the aural signal for each of the active voice frames; and a speech-segment determining section for determining, for each of the active voice frames as an aural signal given in order of time sequence, an accuracy that the active voice frame belongs to an active voice segment by weighting the probability determined by said speech-segment inferring section with the quality monitored by said quality monitoring section.