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 for deciding whether the vocabulary dictionary managing unit uses or not the scale information at the time of extracting the recognition target words, a monitor control unit for monitoring processing states of at least one of the voice input unit, the matching unit and the result output unit, and for outputting an update signal when detecting a given processing state of at least one of the voice input unit, the matching unit and the result output unit, an extraction criterion information managing unit for changing the extraction criterion information stored in the extraction criterion information storing unit based on the update signal according to the detected processing state of at least one of the voice input unit, the matching unit and the result output unit from the monitor control unit, and the monitor control unit further monitors an operation accepted from a user and a measurement unit for measuring a time and outputs the update signal based on at least one of the recognition result of the result output unit, the modification, by the user, of the operation or the input voice, an interval between the voice inputs or the operations by the user and a number of calculating the degree of matching by the matching unit, with the change in the extraction criterion information, the vocabulary dictionary managing unit switches using or not using the scale information for extracting the recognition target words, thereby 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.