Japanese is a language having intonations, which are indicated by the relative differences in pitch heights and the accentual phrases (APs) are placed according to the changes of the accents while a break occurs on a boundary of the APs. Although a break can be predicted by using J-ToBI, which is a rule-based or statistical approach, it is very difficult to predict a break exactly due to the flexibility. Therefore, in this paper, a method which can enhance the quality of synthesized speech by reducing the errors in predicting break indices (BI), are proposed. The method is to use a new definition for the phonetic symbols, which combine the phonetic values of Japanese words with the accents information. Since a stream of defined phonetic symbols includes the information on the changes in intonations, the BI can be easily predicted by dividing the intonation phrase (IP) into several APs. As a result of an experiment, the accuracy of break generations was 98 % and the proposed method contributed itself to enhance the naturalness of synthesized speeches.