A 74-year-old woman was referred to the hospital because of fecal occult blood. Colonoscopic fiber visualized a cecal elevated lesion like a submucosal tumor. Barium enema study demonstrated an intracanalicular smooth elevation surrounded by a bridging fold in the cecum and compression in the terminal ileum. Ultrasonography, CT and magnetic resonance imaging scans of the abdominal and pelvic regions demonstrated a cystic appendiceal mass filled with mucus and an ovarian cyst measuring 6 cm in diameter. Mucinous cyst of the appendix with ovarian metastasis was suspected and rt-hemicolectomy (D3) and bilateral adnexectomies were performed. On histopathological study, mucinous cystadenoma was diagnosed for the appendicial mass, while struma for the ovarian cyst. Mucinous cyst of the appendix concomitant with struma ovarii is quite rare and there have been no previous case reports. Macroscopic differentiation of benignancy and malignancy is difficult for a mucinous cyst of the appendix. Therefore it is appropriate to tentatively diagnose it as malignancy and proceed with surgery if ovarian metastasis is suspected like in this case.