1. A speech recognition system for recognizing the content of an input speech signal comprising: a reference pattern memory for storing reference patterns each being time-serial feature vectors of groups comprising at least first through third formant frequency information for a plurality of analysis frames; first means for developing LPC coefficients for each one of a predetermined plurality of analysis degrees, in each of said plurality of analysis frames, from an input pattern of said input speech signal; second means for developing first through third formant frequencies, for each of said plurality of analysis degrees in each of said plurality of analysis frames, from the developed LFC coefficients; third means for determining distances between said reference patterns and said input pattern, said input pattern being comprised of time-serially connected feature vectors for a plurality of groups of first through third formant frequency information for said plurality of frames, and each of said, groups being one of the groups of first through third formant frequency information developed for said plurality of analysis degrees in each frame; and fourth means for determining the reference pattern having the shortest distance from said input pattern on the basis of the distance determined by said third means as said input pattern.