An extended-word selecting section calculates a score for a phoneme string formed of one more phonemes, corresponding to a user's speech, and searches a large-vocabulary-dictionary for a word having one or more phonemes equal to or similar to those of a phoneme string having a score equal to or higher than a predetermined value. A matching section calculates scores for the word searched for by the extended-word selecting section in addition to a word preliminary word-selecting section. A control section determines a word string as the result of recognition of the speech uttered by the user.