1. A speech recognition apparatus comprising: a speech analyzing unit that converts an inputted utterance speech into a feature amount; a recognized-word storing unit that stores a pronunciation of a recognized word; a conversion rule storing unit that stores a conversion rule between a pronunciation and a phoneme or a conversion rule between a pronunciation and a phoneme string; a phoneme string converting unit that converts the pronunciation of the recognized word stored in the recognized-word storing unit into a phoneme string in accordance with the conversion rule stored in the conversion rule storing unit; a phoneme model storing unit that stores a standard pattern obtained by modeling information about which phoneme tends to become what kind of feature amount; a word model generating unit that generates a word model as a standard pattern string by extracting standard patterns stored in the phoneme model storing unit on the basis of the phoneme string converted by the phoneme string converting unit and by linking the extracted standard patterns; and a speech collating unit that calculates similarities at each time between the feature amount converted by the speech analyzing unit and the word model generated by the word model generating unit, wherein the conversion rule storing unit further stores a first threshold value condition associated with a phoneme or a phoneme string of at least one of the conversion rules, and the speech collating unit extracts a word model from the word models generated by the word model generating unit, whose minimum similarity among the similarities at each time or whose overall similarity obtained from similarities at each time satisfies a second threshold value condition, and whose similarity at each time in a section among vocalization sections of the utterance speech and corresponding to either a phoneme or a phoneme string associated with the first threshold value condition satisfies the first threshold value condition, and outputs the recognized word as a recognition result corresponding to the extracted word model.