This study examined young children's understanding of second-order mental states, which have embedded structures such as beliefs about beliefs. Participants were 4-, 5-, and 6-year-olds. First-order and second-order false belief tasks were prepared with similar structures to minimize the difference between them. The results showed that over half of the 6-year-olds were successful on both tasks, while the 5-year-olds were successful on the first-order but not the second-order false belief task. This time lag between understanding first-order and second-order mental states is consistent with the findings of previous studies. These findings suggest that information processing of embedded mental states is a significant: factor for understanding second-order mental states.

Key words: understanding of second-order mental states, second-order false belief task, young children