1. An apparatus for dividing a main speech of a first speaker in a conversational dialog comprising the first speaker and a second speaker into at least one utterance, the apparatus comprising: a computer processor configured to execute: a pause detecting section for detecting pauses in the main speech of the first speaker received from a first channel among at least two channels; an acknowledgement detecting section for detecting acknowledgements in a speech of the second speaker received from a second channel of the at least two channels, wherein the second channel is separate from the first channel; a boundary-candidate extracting section for extracting boundary candidates in the main speech of the first speaker received from the first channel based, at least in part, on identifying pauses detected by the pause detecting section that are located within a predetermined range before and/or after respective locations of the acknowledgements detected by the acknowledgement detecting section in the speech of the second speaker received from the second channel; and a recognizing unit for outputting a word string associated with at least one utterance formed by segmenting the main speech of the first speaker received from the first channel according to at least one of the extracted boundary candidates.