To enable selection of a speaker, the acoustic feature value of which is similar to that of an utterance speaker, with accuracy and stability, while adapting to changes even when the acoustic feature value of the speaker changes every moment, a long-time speaker score is calculated (log likelihood of each of a plurality of speaker models stored in a speaker model storage with respect to the acoustic feature value) based on an arbitrary number of utterances, for example, and a short-time speaker score is calculated based on a short-time utterance, for example. Speakers are selected corresponding to a predetermined number of speaker models having a high long-time speaker score. Speakers are selected corresponding to the speaker models, the number of which is smaller than the predetermined number and the short-time speaker sore of which is high, from among the speakers having a high long-time speaker score.