1. A speech recognition apparatus where a system that is an object of analysis is provided with a plurality of states, comprising: a code book in which a feature vector space is clustered and representative vectors of respective clusters are stored in a form searchable by labels thereof; cluster occurrence probability storing means for storing occurrence probabilities of the respective labels in the respective states; membership degree calculating means for calculating membership degrees of observation vectors to the respective clusters by using the code book; and observation vector occurrence degree calculating means for calculating a sum of products of (a) the calculated membership degrees of the observation vectors to the respective clusters and (b) logarithmic values of the occurrence probabilities of the respective clusters stored in the cluster occurrence probability storing means, or an amount equivalent thereto, wherein the sum of products corresponds to occurrence degrees of observation vectors in the respective states of the system, and the cluster occurrence probability storing means calculates the occurrence probabilities of the clusters such that, for a predetermined value N, the occurrence probabilities with respect to the clusters in which the occurrence probabilities at an (N+1)-th order or lower are rendered a non-zero common value, and a total sum of the occurrence probabilities of the clusters is 1.