The present study analyzes code-switching in Korean and Japanese by observing instant messaging group chat data involving Japanese teachers and advanced Japanese language learners. Although there have been some preceding studies on code-switching that occurs in SNS texts, the present study aims at revealing the characteristics of Hangeul transcriptions of Japanese by focusing on the switching of the transcription language rather than the switching of the language itself. The results of the analysis are as follows. First, intra-sentential code-switching characteristics in common with preceding studies include the transcription of loanwords in imitation of the Japanese pronunciation and the use of combination forms involving Japanese and ‘-hada’ on the one hand and Korean and ‘-desu’ on the other. The study differs from preceding studies in that code-switching occurs in both Korean and Japanese base sentences. Also, complex code-switching occurs in sentences. This may be classified as conversational code-switching. Involving simultaneous communication, instant messaging is closer to actual conversation than Facebook or Instagram, so the characteristics of code-switching more easily manifest in instant messaging as these characteristics appear in conversation. Next, the functions for transcribing Japanese with Hangeul involve economy, entertainment, and group consciousness. In the future, research is expected to examine the aspects and frequency of code-switching by each learner and the correlation between Japanese language learning duration and code-switching.