This study explores a possible timing relationship between speech units in Japanese and their mapping onto musical units, in which an attempt is made to examine rhythmic patterning in language and its interaction with musical rhythm. Three questions are set out; what is the Japanese speech unit to determine spoken rhythm; is the unit altered in musical constraint; has the pattern changed over time? Various methods were employed to examine the questions such as analyses on songs for children, a set of experiments with native speakers of Japanese, and acoustic analyses of spoken/sung utterances. Throughout all the data analysed, we obtained a set of consistent results. (1) Subjects showed their sensitivity to both single moraic units and larger units. (2) Musical structures do not constrain linguistic rhythm. (3) The younger subjects preferred larger units as a timing unit. (4) Phonetic features are evident if observation was made at smaller units, but musical structure dominates in larger units. The findings obtained here reflect a strong correlation of rhythmic patterning between spoken and sung utterances in the context of intra-language.