This work investigates the constraints on the usage of theJapanese particle mo and its discourse functions. The author claims that the usage of mo is constrained by the relation between the proposition within the scope of the particle (the 'host proposition' hereafter) and the context, defining 'context' as a set of propositions that the speaker assumes to be shared by the hearer. The analysis shows that, for the host proposition F(x) where x is the constituent marked by mo, the context must include an antecedent proposition H(y) such that y is not x and that F(x) entailsH(x) such that H(x) is 'contextually relevant' to the context.