1. A speech recognition device for recognizing speech of unspecified speakers using hidden Markov models, the device comprising: detection means for detecting feature parameters of input speech; data storage means for storing transition probabilities and output probability functions, which use, as arguments, said feature parameters for multiple predetermined types of hidden Markov models, the models each representing a plurality of predetermined words; and recognition means for determining the occurrence probability that a sequence of said feature parameters will occur using said hidden Markov models, said recognition means assigning each of said words a state sequence of one hidden Markov model common to said multiple types of hidden Markov models, the occurrence probability determined by selecting a largest product of a transition probability with an output probability function associated with each state of said common hidden Markov model, wherein the input speech is recognized based on the occurrence probability thus determined.