The contrast of the vowel length in Modern Japanese results from the coalescence in Vu hiatuses as [iu][eu][au][ou], which changed into long vowels. This paper intends toexplain what factors were relevant to these changes. It presents two points as follows.
(1) As a result of the above-mentioned changes, i) the indivisible units that had two morae, i.e. the long vowels, came to be established. ii) the syllable u came to be restricted to be at the initial positions of morphemes. These processes are worthy of notice from the functional point of view. They are regarded consonant with the trend, which has been already pointed out, of the phonological evolution in the history of Japanese language: that the delimitation of morphemes has been manifested by the phonotactic and tonal rearrangement resulting from several different changes.
(2) The manner of changes in Vu and in Vi was conditioned by the phonetic nature of Vu and of Vi. Vu hiatuses were vulnerable to the changes into long vowels under the condition that the [j] follows all consonants except [w].