1. A voice search device comprising: a processor; and a memory storing instructions that, when executed by the processor, control the processor to: convert a search string into a phoneme sequence; acquire durations of respective phonemes included in the phoneme sequence; derive a spoken time length of voice corresponding to the search string based on the durations; designate a plurality of designated zones having time lengths in a target voice signal; acquire, using an acoustic model that does not depend on adjacent phonemes, a first group of likelihoods indicating how likely each zone from among the plurality of designated zones is a zone in which voice corresponding to the search string is spoken; specify, based on the first group of likelihoods, a plurality of estimated zones from among the plurality of designated zones, wherein each estimated zone is a zone in which the voice corresponding to the search string is estimated to be spoken, and wherein a number of the estimated zones is less than a number of the plurality of designated zones; and acquire, using an acoustic model that depends on adjacent phonemes, a second group of likelihoods indicating how likely each of the plurality of estimated zones is a zone in which the voice corresponding to the search string is spoken.