The rules of properly writing kana in Japanese are called “Kana-tsukai.” In general, these “Kana-tsukai” rules dictated that successive phonemes be written with different forms of the phoneme, but the general public largely ignored this in actual usage. Moreover, it is worthy of attention whether the “Kana-tsukai” for the phoneme /tsu/ following vocal sounds was differentiated or not. This is because though the notations for /tsu/ originally transcribed the combination of a vowel and a consonant (“tsu”), later this phoneme was also used to transcribe /Q/. /Q/ means double consonant in Japanese. However, because people at that time did not seem to be interested in differentiating usage of the characters between phonemes, one cannot study the method of usage separating /tsu/ and /Q/. Regardless, there are some examples where /tsu/ and /Q/ are differentiated in the texts. For this reason, the present study examines the characteristic use of /tsu/ in texts written by Japanese people that relate to the unique genre of Korean texts from the late Middle Ages. It is clear that there is an intention especially in the Korean texts to use /tsu/ to represent the syllable “tsu,” but also there is an intention to use the /tsu/ to represent codas, aspect consonants, or intensive consonants. However, it does not seem as though there was any intention to differentiate aside from simply emulating Japanese writing customs. This aspect continued through the ages, and gradually change appeared. Rather than showing a clear differentiation of use, this paper shows that “kosen” (an arc) or “kenten” (a circle) function to separate /tsu/ and /Q/. And this paper shows that this transcription functions to transcribe more than just spoken Korean, but also Japanese Korean language as well.