The understanding of sarcasm is important for successful communication in interpersonal relationships. Previous studies have pointed out that the ability to understand sarcasm develops from childhood to adolescence. In particular, previous behavioral studies have shown that, while sixth grade children could understand sarcasm with intonation as well as adults, their understanding of sarcasm without intonation was lesser than that of adults. However, the neural basis underlying development of this ability remains unknown. To investigate this issue, we conducted functional magnetic resonance imaging of sixth grade children and adults while they performed a visually presented sarcastic discourse task. Thirty-five healthy adult subjects and nineteen healthy sixth grade subjects participated in the present study. Whole brain analysis of adults revealed activation in the left middle frontal gyrus, medial frontal gyrus, right cerebellum, and left intraparietal gyrus. In contrast, we did not detect any sarcasm-related activation in the sixth grade subjects using this analysis. Furthermore, we conducted a region of interest analysis of the four regions that showed activation in the adults. There were no significant differences between children and adults in the activation of the left middle frontal gyrus or right cerebellum. However, the children displayed less activation in the medial frontal gyrus and left intraparietal gyrus. The medial frontal gyrus and parietal region are known to be involved in the mentalizing processing. These results suggest that the sixth grade children are still developing the neural processes to understand sarcasm. They may be processing sarcasm using a different way compared to adults, who use the mentalizing system.