1. A speech recognition system comprising: a first microphone for picking up input speech signals; a speech-signal-duration determining unit for determining speech signal durations by monitoring the amplitude of the input speech signals; a recognition unit for determining recognition results by analyzing and recognizing signals given in said speech signal durations; a second microphone provided to pick up ambient noises; means for measuring the intensity of input signals given from said second microphone; and a rejection unit which, when the intensity of said noise exceeding a predetermined standard value is detected in any of said speech signal durations, rejects the recognition results determined by said recognition unit.