Kindergarten children of ages three through five as well as adults pronounced six Japanese nouns including long consonats, syllabic nasals, and long vowels. Their utterances were recorded and segmental durations were measured based mainly on the spectrographic analysis. The results showed that a strong developmental change is observed in the acquisition of long consonants and syllabic nasals whereas a weak developmental change is observed in the acquisition of long vowels. They also showed that no effect of word accent is observed in the acquisition of these sounds.