1. A speech recognition apparatus comprising: a memory; and a processor executing a plurality of instructions stored in the memory, the instructions comprising: a speech analyzing instruction that converts an inputted utterance speech into a feature amount; a recognized-word storing instruction that stores a pronunciation of a recognized word; a conversion rule storing instruction 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 instruction that converts the pronunciation of the recognized word stored by the recognized-word storing instruction into a phoneme string in accordance with the conversion rule stored by the conversion rule storing instruction; a phoneme model storing instruction that stores a standard pattern obtained by modeling information about which phoneme tends to become what kind of feature amount; a word model generating instruction that generates a word model as a standard pattern string by extracting standard patterns stored by the phoneme model storing instruction on the basis of the phoneme string converted by the phoneme string converting instruction and by linking the extracted standard patterns; and a speech collating instruction that calculates similarities at each time between the feature amount converted by the speech analyzing instruction and the word model generated by the word model generating instruction, wherein: the conversion rule storing instruction further stores a first threshold value condition associated with each of ambiguous phonemes or each of ambiguous phoneme strings of at least one of the conversion rules, which is ambiguous in a case where a human being vocalizes the recognized word, wherein different values are assigned to said first threshold value condition for different ambiguous phonemes and/or different ambiguous phoneme strings, and the speech collating instruction extracts a word model from the word models generated by the word model generating instruction, whose minimum similarity among the similarities at each time satisfies a second threshold value condition, and whose overall similarity obtained from similarities at each time satisfies a third threshold value condition, and whose similarity at each time in a section among vocalization sections of the utterance speech and corresponding to either one of the ambiguous phonemes or one of the phoneme strings, satisfies the first threshold value condition, which is stored by being associated with the one of the ambiguous phoneme or the one of the ambiguous phoneme strings, and outputs the recognized word as a recognition result corresponding to the extracted word model.
4. The speech recognition apparatus according to claim 1 , further comprising: an inputting instruction that accepts a decision from a user whether the recognition result outputted by the speech collating instruction is incorrect or not; a use-frequency calculating instruction that, when the inputting instruction accepts from the user a decision that the recognition result outputted by the speech collating instruction is incorrect, calculates a use-frequency of the conversion rule used for generating the word model corresponding to the recognized word as the recognition result; and a first threshold value condition updating instruction that, when a use-frequency of the conversion rule calculated by the use-frequency calculating instruction is higher than the boundary condition, updates a first threshold value condition associated with the phoneme or the phoneme string of the conversion rule so that the first threshold value condition is raised, and when the use-frequency of the conversion rule calculated by the use-frequency calculating instruction is lower than the boundary condition, updates the first threshold value condition associated with the phoneme or the phoneme string of the conversion rule so that the first threshold value condition is lowered.