1. A speech recognition method of recognizing an input speech in a noisy environment by using a plurality of clean speech models, each of said clean speech models having a clean speech feature parameter S representing a cepstrum parameter of a clean speech thereof, said speech recognition method comprising the processes of: detecting a noise feature parameter N representing a cepstrum parameter of a noise in said noisy environment, immediately before said input speech is input; detecting an input speech feature parameter X representing a cepstrum parameter of said input speech in said noisy environment; calculating a modified clean speech feature parameter Y according to a following equation: EQU Y=k.multidot.S+(1-k).multidot.N (0&lt;k.ltoreq.1), where said "k" is a predetermined value corresponding to a signal-to-noise ratio in said noise environment; comparing said input speech feature parameter X with said modified clean speech feature parameter Y; and recognizing said input speech by repeatedly carrying out said calculating process and said comparing process, respectively, with respect to said plurality of clean speech models.