The purpose of the present study was to investigate the preschoolers' understanding of an other's cognitive (epistemic) content. In Exp. 1, fifty subjects (3-, 4-, and 5-year-olds) were asked about a story character's false belief and were required to justify such a cognitive state. In Exp. 2, thirty-eight subjects (4-, and 5-year-olds) had their understanding of a sudden change of other's cognitive content tested. Consequently, the following levels of understanding of a false belief were found. In Level 1; Subjects could understand other's false belief but were not able to be aware of the causal relation between other's perception/action and his/her knowledge. In Level 2; Subjects were able to be aware of such a ralation. And the latter could understand the change of other's cognitive content as well.