This paper investigates the ambiguity of tense in the Japanese mirative sentence with nante/towa. Unlike an English sentence exclamative (e.g., (Wow), John won the race!), a Japanese sentence with nante/towa has a property of ambiguity with regard to tense. When nante or towa is combined with a proposition that contains the so-called non-past form ru, the sentence can be ambiguous between a non-past (future/present) reading and a past reading. This fact is surprising because the non-past form ru can never be used for describing a past event. We argue that the ambiguous interpretation of nante/towa comes from the conventional implicature of nante/towa. Unlike an English sentence exclamation (Rett 2011), the Japanese nante/towa takes a “tenseless” proposition p (i.e., ru does not specify a tense) and conventionally implies that (i) p is settled (i.e., p is/was true or predicted to be true) and (ii) the speaker had not expected that p. We will also consider the case where p + nante/towa is embedded under a surprising predicate and claim that both the embedded and non-embedded nante/towa can be analyzed in a uniform way, suggesting that the embedded nante/towa clause is an instance of a main clause phenomenon (rather than a relative tense phenomenon).