This paper explores the interplay of (inter)subjectivity and social norm. (Inter)subjectification is a diachronic process, strengthening the speaker’s (inter)subjective meanings. However, when language change, including (inter)subjectification, occurs, what roles do society or any other social factor play in such change? To address this question, the paper suggests a specific mechanism behind the speaker’s choice of linguistic forms. As episodes exemplifying intersubjectification, the meaning shifts of Japanese “involvement markers”, na elements, are examined. Their meaning shifts include: (1) from “self-addressed” (subjective) to “other-addressed” (intersubjective) meanings; and (2) from intersubjective to more intersubjective meanings. The (inter)subjective conversational strategies with the use of na elements contribute to fulfill one Japanese social norm, “harmony (wa)” (Ide and Kataoka 2002: v; Nakane 1970: 49). In this paper, the close connection between intersubjectivity and social norm is also shown, being supported by a classic cross-linguistic study of European T-V languages (Brown and Gilman 1960) and a cross-linguistic analysis of Korean and Japanese intersubjectification.