(a) obtaining a first segment set, the set including a phoneme environment, address data of each segment of respective languages, and segment data of each segment, which are corresponding with each other;

(b) converting a plurality of sets of phoneme labels defined in each language into a common set of phoneme labels shared by the multiple languages;

(c) converting a plurality of sets of prosody labels defined in each language into a common set of prosody labels shared by the multiple languages;

(d) creating triphone models from a speech database for training;

(e) creating a decision tree using the triphone models and a set of questions relating to the phonological environment, the phonological environment including a phoneme environment represented by the common set of phoneme labels and prosody environment represented by the common set of prosody labels;

(f) performing clustering of the first segment set using the decision tree;

(g) for each cluster obtained in step (f), selecting a template segment having the maximum time length of the largest number of pitch periods of the segments belonging to a cluster;

(h) deforming the segments belonging to the cluster to have the number of pitch periods and the pitch period length of the template segment;

(i) generating a representative segment of a segment set belonging to the cluster by calculating an average of the deformed segments;

(j) for each cluster, replacing segments belonging to the cluster with the representative segment and deleting segment data of the replaced segments; and

(k) creating a second segment set as an updated set of the first segment set by replacing the address data of each replaced segment with address data of a corresponding representative segment.