Central vowels are observed both in the Ryukyuan dialects and East-northern dialects of the Japanese language. This paper takes notice of the u-fronting preceded by the coronal consonants which were observed in the 16th century dialect of Okinawa main island. It is speculated that this kind of coarticulation also was a trigger change toward the centralization of [u] vowel of East-northern dialects of Japanese. Just as the coronal consonants influence the shape of the tongue body by u-fronting, it is also supposed that the shape of the tongue pronouncing [i] could also be assimilated to the preceding coronal consonants. As the results of the above two assimilations, firstly /i/ and /u/ are speculated to have begun centralization in the environment following /s, z, c/. Secondly the consonants preceding central vowels have extended to all the consonants other than /s, z, c/, and thirdly /i/ and /u/ following /s, z, c/ have ceased to be distinct phonemically.