Bibliogr. s. 118

"Speech level shift" is a phenomenon in which formal speech is used in casual conversations. This paper aims to clarify when and why this temporary shift occurs and considers whether its functions are unique to the Japanese language by employing the concept of "politeness theory". According to this theory, a speaker has two choices of strategy for successful communication. The first, namely "positive politeness strategy", attempts to decrease their distance with the listener, while the second, "negative politeness strategy", tries to maintain this distance. Using examples from a database of Japanese conversations, four functions of speech level shift based on politeness strategies were found. When the shift appears at the beginning or the end of a topic, its function is to reduce the suddenness of conversational transition, while in the case of communicating blame, criticism, or requests, the shift has the function of softening something hard to say. These two functions are based on negative politeness strategy. When the shift is used to emphasize something important in the conversation, its function is to make the speech clearer, while in the case of telling a joke, the shift can create an emotional connection through humor. These two functions are based on positive politeness strategy. In this way, speech level shifts in the Japanese language have four principal functions, which are explainable by theories developed internationally. This fact shows that there is a commonality between Japanese and other languages regarding the function of consideration for others.