5. The voice recognition apparatus according to claim 1 , further comprising a use frequency managing unit for monitoring the number of uses of each of the plurality of words stored in the vocabulary dictionary storing unit and calculating a use frequency of each of the plurality of words, a use frequency storing unit for storing, as use frequency data, the use frequency calculated by the use frequency managing unit so as to correspond to each of the plurality of . words stored in the vocabulary dictionary storing unit, and a scale information managing unit for updating the scale information stored in the scale information storing unit using at least the use frequency data stored in the use frequency storing unit.
7. The voice recognition apparatus according to claim 5 , further comprising a vocabulary group use frequency managing unit for dividing the plurality of words stored in the vocabulary dictionary storing unit into a plurality of vocabulary groups and calculating a use frequency of each of the vocabulary groups based on the use frequency of each of the plurality of words belonging to the vocabulary group stored in the vocabulary dictionary storing unit, a vocabulary group use frequency storing unit for storing, as vocabulary group use frequency data, the use frequency of the vocabulary group calculated by the vocabulary group use frequency managing unit so as to correspond to each of the vocabulary groups, and a threshold storing unit for storing a threshold indicating a criterion of the vocabulary group use frequency data at the time of extracting the recognition target words, wherein the vocabulary dictionary managing unit selectively performs one of operations (3) and (4) below referring to the threshold stored in the threshold storing unit and the vocabulary group use frequency data stored in the vocabulary group use frequency storing unit according to the extraction criterion information stored in the extraction criterion information storing unit: (3) for the vocabulary group whose vocabulary group use frequency data are equal to or larger than the threshold, the vocabulary dictionary managing unit extracts all the words belonging to this vocabulary group as the recognition target words regardless of the scale information, (4) for the vocabulary group whose vocabulary group use frequency data are smaller than the threshold, the vocabulary dictionary managing unit extracts the recognition target words from the words belonging to this vocabulary group based on the scale information.