This paper investigates the usage of informal-style (IF) by advanced learners of Japanese as a second language when they shift from a formal (polite) speech style into an informal (plain) version. The data consists of fifteen dyads between native and non-native speakers who meet for the first time. The results show there was negligible difference in the plain form usage between learners and native speakers, but the learners used IF proportionally more than native speakers. The analysis of sentence final particles, rising intonation, and WH-questions in the learners’ IF forms revealed that the learners’ use of plain forms became IF in the following two occasions: First, the learners expressed a spontaneous emotional reaction, and the plain form was accompanied by the particle YO, NE, or rising intonation. Second, they used rising intonation but did not use self-monitoring discourse markers in monologues.