1. A speech recognition apparatus for recognizing speech by comparing composite acoustic models adapted to noise and speaker with a feature vector series extracted from an uttered speech, comprising: a storing section for previously storing each representative acoustic model selected as a representative of acoustic models belonging to one of groups, each of said groups being formed beforehand by classifying a large number of acoustic models on a basis of a similarity, difference models of each group obtained from difference between said acoustic models belonging to one of said groups and said representative acoustic model of said identical group, and group information for corresponding said representative acoustic models with said difference models every said identical group, a generating section for generating each noise adaptive representative acoustic model of said each group by noise-adaptation executed to said representative acoustic model of said each group stored in said storing section; a generating section for generating each composite acoustic model of said each group by composition of said difference model and said noise adaptive representative acoustic model using said group information; a renewal model generating section for generating noise and speaker adaptive acoustic models by performing a speaker-adaptation of said composite acoustic model every identical group, using the feature vector series obtained from the uttered speech; and a model renewal section for replacing said difference models of said each group by renewal difference models which are generated by taking differences between said noise and speaker adaptive acoustic models and said noise adaptive representative acoustic models selected via said group information; wherein a speech recognition is performed by comparing the feature vector series extracted from the uttered speech to be recognized with said composite acoustic model adapted to noise and speaker, and wherein said composite acoustic model adapted to noise and speaker is generated by composition of said renewal difference model and said noise adaptive representative acoustic model, which is generated by a noise-adaptation of said representative acoustic model of said group including said renewal difference model selected via said group information.