output probabilities are made only of the top n states having the highest output probabilities however if a plurality of types of hmms for example a rough hmm a comparatively detailed hmm and a more detailed hmm are employed the following processing stages may be used namely the output probabilities of all the states may first be determined by use of the rough hmm then the output probabilities of a range from the top a states to the top b states a lt b b a 1 having the highest output probabilities obtained by the previous determinations may be redetermined by the comparatively detailed hmm the output probabilities of the top a 1 states may be similarly redetermined by use of the more detailed hmm according to this process in a plurality of stages the most precise output probability can be achieved when a plurality of types of hmms that is a rough hmm a comparatively detailed hmm and a more detailed hmm are used the following computation stages may be used alternatively the output probabilities of all the states may first be determined by use of the rough hmm then the output probabilities of the top b states having the highest output probabilities obtained by the previous determinations may be redetermined by use of the comparatively detailed hmm further the output probabilities of the top a states a lt b having the highest output probabilities obtained by the previous redeterminations may be.