A system for voice detection includes a feature value calculation unit that calculates a feature value from an input signal sliced on a per frame basis, a provisional voice/non-voice decision unit that provisionally decides a voiced interval and a non-voiced interval from the feature value calculated on a per frame basis, and a voice/non-voice decision unit that determines a voiced interval duration threshold value or a non-voiced interval duration threshold value, using a ratio of the feature value found on a per frame basis to a threshold value for the feature value and that re-decides the voiced interval and the non-voiced interval, using the voiced interval duration threshold value determined and the non-voiced interval duration threshold value determined. By determining the voiced interval duration threshold value and the non-voiced interval duration threshold value, using the feature value found on a per frame basis and the threshold value for the feature value, the constraint of the shaping rule may be made weaker, or stronger in case the feature value found on a per frame basis can be regarded as being reliable or not, thereby allowing voice detection to be made without dependency upon a noise environment.