The aim of this study is to clarify the process of the sound changes of Japanese consonants (Dakuons) in the analysis of the transcriptions of Korean Source-Materials (i.e. Japanese textbooks for Korean and the records of travel in Japan) written in the 15-l8th centuries with the Korean writing system. Especially these records of travel in Japan are meaningful in that the process of change of Dakuon is shown in detail. The results are as follows. 1) In the 15th century, all Dakuons /g d z b/ had nasality. 2) The nasality of /z/ and /b/ disappeared in the 16th century. 3) The nasality of /d/ disappeared in the late 17th century.