A search string acquiring unit acquires a search string. A converting unit converts the search string into a phoneme sequence. A time length deriving unit derives the spoken time length of the voice corresponding to the search string. A zone designating unit designates a likelihood acquisition zone in a target voice signal. A likelihood acquiring device acquires a likelihood indicating how likely the likelihood acquisition interval is an interval in which voice corresponding to the search string is spoken. A repeating unit changes the likelihood acquisition zone designated by the zone designating unit, and repeats the process of the zone designating unit and the likelihood acquiring device. An identifying unit identifies, from the target voice signal, estimated intervals for which the voice corresponding to the search string is estimated to be spoken, on the basis of the likelihoods acquired for each of the likelihood acquisition zones.