1. A noise adaptation system of speech model for adapting a speech model for any noise to speech to be recognized in a noisy environment, said speech model being learned by using noise data stored in a noise database and clean speech data, said system comprising: clustering means for clustering all noise data stored in said noise database; speech model space generating means for generating a single-tree-structure noisy speech model space based on the result of the clustering performed by said clustering means; parameter extracting means for extracting a speech feature parameter of input noisy speech to be recognized; selecting means for selecting an optimum model from the tree-structure noisy speech model space generated by said speech model space generating means; and linear transformation means for applying linear transformation to the model selected by the selecting means so that the model provides a further increased likelihood.