1. An information search method using an Inverse Hidden Markov Model (IHMM),for stochastically searching for a reference information model among a plurality of predetermined reference information models obtained by training that best matches unknown information which is expressed by a HMM chain, the information search method comprising the steps of: (a) obtaining a minimum state probability &phgr;′ i (t) for a particular state searched at a current time t in a HMM state lattice of a reference information model by using minimum state probabilities of effective states, each of which is a probability accumulated along a search path up to a previous time t−1, and then updating the obtained minimum state probability &phgr;′ i (t) with a predetermined value if the minimum state probability &phgr;′ i (t) is greater than a minimum unlikelihood score; (b) obtaining an optimal path value corresponding to the lowest of a plurality of minimum state probabilities obtained after step (a) is performed for a period of time T which is greater than t, and updating the minimum unlikelihood score with the optimal path value if the minimum unlikelihood score is greater than the optimal path value; (c) determining whether or not the optimal path value is obtained for each of the predetermined reference information models, and if not, iterating steps (a) and (b) for another reference information model for which the optimal path value has not been obtained; and (d) if the optimal path value has been obtained for each of the reference information models, determining that the reference information model yielding the optimal path value with which the minimum unlikelihood score was last updated best matches the unknown information, wherein for the period of time T, step (a) is performed for each of one or more states searched at every current time t&equals;1, 2,... T, the effective states are states searched at previous time t−1 which have minimum state probabilities less than the minimum unlikelihood score, and from which a transition can be made to the particular state searched at time t, and the minimum state probability of the particular state corresponds to the lowest among state probabilities of the particular state for making a transition to the particular state from the effective states.