This study examined the age-related changes in children’s understanding of second-order beliefs, and assessed the effect of task simplification (i.e. the reduction of information-processing demands of the false belief task) on such understanding. Five-, 7-, and 9-year-old children (N=78) performed three second-order false belief tasks. Two standard tasks were a Perner and Wimmer(1985)’s story and a Sullivan, Zaitchik, and Tager-Flusberg (1994)’s story. A new task was made simpler in its story structure by reducing the number of characters and scenes, and the story length. All tasks included probe and feedback procedures, which Sullivan et al. proposed for enhancing young children’s performance. The findings were: (1) between 5 and 7 years of age, children underwent a significant developmental change in their second-order understanding, (2) 5-year-old children who appeared to fail the standard tasks performed significantly better on the new, simplified task, showing that 65% of them were able to successfully attribute second-order false beliefs. These findings suggest that at age 5, children can acquire the second-order understanding in a simplified false belief task, and also that task simplification can facilitate children’s acquisition of such understanding.