It has been repeatedly shown that when asked to identify a protagonist's false belief on the basis of his false statement, English-speaking 3-year-olds dismiss the statement and fail to attribute to him a false belief. In the present studies, we tested 3-year-old Japanese children in a similar task, using false statements accompanied by grammaticalized particles of speaker (un)certainty, as in everyday Japanese utterances. The Japanese children were directly compared with same-aged German children, whose native language does not have grammaticalized epistemic concepts. Japanese children profited from the explicit statement of the protagonist's false belief when it was marked with the attitude of certainty in a way that German children did not – presumably because Japanese but not German children must process such marking routinely in their daily discourse. These results are discussed in the broader context of linguistic and theory of mind development.