1. A speech recognition method in which an input speech in converted to a time sequence of a feature vector, said feature vector including one of a spectrum and a cepstrum, and a distance or probability between the input speech time sequence and a time sequence of the feature vector or a statistical model thereof, is calculated for recognition, comprising the steps of: effecting a time frequency masking by an operation of obtaining a masked speech spectrum by subtracting, from speech spectrum at present, a masking pattern which is a function of frequency obtained by smoothing immediately preceding speech spectrum by time and frequency; and recognizing the speech by using the masked speech spectrum obtained by the above described operation at every time point.