1. A voice recognition apparatus comprising: a voice input unit for converting an accepted voice into digital voice data; a voice analyzing unit for converting the digital voice data into a feature value; a vocabulary dictionary storing unit for storing a plurality of words in advance; a vocabulary dictionary managing unit for extracting recognition target words, which are targeted for recognition, from the plurality of words stored in the vocabulary dictionary storing unit; a matching unit for calculating a degree of matching with the recognition target words extracted by the vocabulary dictionary managing unit using the feature value converted by the voice analyzing unit; and a result output unit for outputting, as a recognition result, a word having a best score from a result of calculating the degree of matching by the matching unit; wherein the voice recognition apparatus comprises a scale information storing unit for storing in advance scale information serving as a scale for extracting the recognition target words for each of the plurality of words stored in the vocabulary dictionary storing unit, an extraction criterion information storing unit for storing in advance extraction criterion information indicating a criterion of the scale information at the time of extracting the recognition target words, a monitor control unit for monitoring predetermined states, and an extraction criterion information managing unit for changing the extraction criterion information stored in the extraction criterion information storing unit according to a result of the monitoring by the monitor control unit, and with the change in the extraction criterion information, the vocabulary dictionary managing unit increases or decreases the number of the recognition target words to be extracted from the plurality of words stored in the vocabulary dictionary storing unit.