1. A voice search device comprising: a search string acquirer acquiring a search string; a converter converting the search string acquired by the search string acquirer into a phoneme sequence; a time length deriver acquiring a duration of each phoneme included in the phoneme sequence converted by the converter, and deriving a spoken time length of voice corresponding to the search string based on the acquired durations; a zone designator designating a likelihood acquisition zone that is a zone of the time length derived by the time length deriver in a target voice signal; a likelihood acquirer acquiring a likelihood indicating how likely the likelihood acquisition zone designated by the zone designator is a zone in which voice corresponding to the search string is spoken; a repeater changing the likelihood acquisition zone designated by the zone designator, and repeating a process of the zone designator and the likelihood acquirer; and an identifier identifying, on the basis of the likelihood acquired by the likelihood acquirer for each likelihood acquisition zone designated by the zone designator, from the target voice signal an estimated zone for which the voice corresponding to the search string is estimated to be spoken.