To obtain the rule capable of accurately determining the phoneme duration time length of mora phonemes, such as syllabic nasal and geminative consonants, and phoneme systems, such as vowel unvoiced phoneme systems and eventually to easily obtain more natural synthesized speeches.

A block and segment part 40 determines the phoneme blocks by segmenting the phoneme symbol strings in the uttered vowels. A section 42 for setting the length of the time between the energy centroids of the vowel parts determines the length DG of the time between the energy centroids of the vowel parts to the phoneme section with the kinds and utterance speeds of the phoneme system held between the vowel parts at both ends of the phoneme section as parameters by using the rhythm rule (A) 44. A section 46 for setting the duration time length of the respective phonemes segments the phoneme blocks to respective phoneme parts and determines the phoneme duration time length of the respective phoneme parts with DG and the kinds of the phoneme systems as parameters by using the rhythm rule (B) 48. The kinds of the phoneme systems are discriminated by the kinds of the consonants following, for example, the syllabic nasal, the kinds of the consonants constituting the geminative consonants and the vowel unvoiced phoneme systems.