1. A speech recognizing apparatus comprising: a speech section detecting unit to detect a speech section from an input signal; a characteristic amount extracting unit to analyze an input speech, which is the input signal in said speech section, and extracting a time series of the amount of characteristics representing characteristics of the input speech; a recognizing target vocabulary storing unit to store predetermined recognizing target vocabularies; a recognizing target vocabulary comparing unit to compare the time series of the amount of characteristics with respective recognizing target vocabularies read from the recognizing target vocabulary storing unit one by one to obtain a likelihood that respective recognizing target vocabularies coincide with the time series of the amount of characteristics; an environment adaptive noise model storing unit to store an environment adaptive noise models adapted to an environmental noise; an environment adaptive noise model comparing unit to compare the time series of the amount of characteristics with respective environment adaptive noise models read from the environment adaptive noise model storing unit one by one to obtain a likelihood that respective environment adaptive noise models coincide with the time series of the amount of characteristics; a rejection determining unit to determine whether or not the input signal is a noise by comparing the likelihood of the registered vocabulary obtained by said recognizing target vocabulary comparing unit with the likelihood of the environmental noise obtained by said environment adaptive noise model comparing unit; and a noise model adapting unit to update the environment adaptive noise model so as to adapt to the input signal when said rejection determining unit determines that the input signal is the noise.