A speech-duration detector includes a starting-end detecting unit that detects a starting end of a first duration where the characteristic exceeds a threshold value as a starting end of a speech-duration, when the first duration continues for a first time length; a trailing-end-candidate detecting unit that detects a starting end of a second duration where the characteristic is lower than the threshold value as a candidate point for a trailing end of speech, when the second duration continues for a second time length; and a trailing-end-candidate determining unit that determines the candidate point as a trailing end of the speech-duration, when the second duration where the characteristic exceeds the threshold value does not continue for the first time length while a third time length elapses from measurement at the candidate point.