This paper aims at clarifying the semantics of Japanese sentential adverb mochiron. In the previous research, mochiron is equated with English expression ‘needless to say’, and usually translated as ‘of course’. More specifically, mochiron indicates the speaker’s acknowledgement of -and agreement on-the hearer’s belief or expectation. In this study, however, I argue that the hearer’s belief is not essential in order to use mochiron. As an alternative account, I propose that the proposition p of ‘mochiron p’ is inferred from two premises: p’ and a hidden reason r. That is, the speaker assumes that p’ is shared by all discourse participants as a common ground, but p is not because r may not exist in hearer’s knowledge and belief. As a result, the speaker and hearer may have different belief about p (p or not p), and the speaker expresses p with mochiron in order to convey the idea that p is inferred from p’ and reason r.