Mucocele of the appendix was first described and named "Hydrops processus vermiformis" by Rokitansky in 1866. Intussusception of a appendiceal mucocele is very rare and we have been able to find only 14 previously reported cases. We present two cases with preoperative ultrasonography which is valuable for its diagnosis. Case 1: A 5-year-old male was admitted to Kahoku Hospital because of right lateral colic abdominal pain and tumor. Ba-enema examination revealed intussusception to the colon and ultrasonography showed the cystic mass of the appendix. Case 2: A 51-year-old female was admitted because of right lower abdominal pain. Ultrasonography was helpful showing the cystic tumor at the base of the appendix. Recent reports say that ultrasonography is valuable examination for its diagnosis. Also in our two cases preoperative ultrasonography was performed and gave us valid information for its qualitative diagnosis.