The similarity between the Chinese unrounded apical front vowels and the Japanese /ウ/vowels is drawing attention. In this article, the history of the transcriptions of Japanese /ス,ズ,ツ,ヅ/ can be explained by the transcriptions of Chinese characters in the Ming dynasty's Chinese documents and the Muromachi era’s transcriptions can be also speculated. Meanwhile, by the facts that the Japanese /ス,ズ,ツ/ transcriptions in the Tō-on documents imitated the Chinese unrounded apical front vowels and the fact that Chinese transcribed the Japanese /ス,ズ,ツ/ by the Chinese unrounded apical front vowels in the Meiji era, the historical fact that Chinese have recognized the Japanese /ウ/vowels as the unrounded apical front vowels and Japanese have recognized the Chinese unrounded apical front vowels as the Japanese /ウ/vowels is drawn. Furthermore, this phenomenon can be seen as an indication of un-rounding of the Japanese /ウ/vowels, and it also clarifies that understanding of the history of devoicing of the vowels which is strongly related to un-rounding of the vowels is an importent point of issue. In the point of the Korean documents, it is also clarified.