An information search method and apparatus employ 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 Hidden Markov Model (HMM) chain. The method and apparatus find an optimal path in a HMM state lattice using a minimum unlikelihood score, rather than a maximum likelihood score, and using a Viterbi algorithm, to recognize unknown information, so that unnecessary computations are avoided. The method and apparatus can be used for finding the most likely path through a vocabulary network for a given utterance.