Mucinous neoplasms occur rarely in association with cystic teratoma, Sertoli-Leydig cell tumor, granulosa cell tumor or carcinoid tumor. Several cases of an ovarian stromal tumor with minor sex-cord elements have been reported in the literatures. However, there has been no report about an ovarian mucinous neoplasm coexisting with a stromal tumor with sex-cord elements yet. We report a case of an ovarian neoplasm composed of both mucinous cystadenoma and stromal tumor with minor sex-cord elements in a 58-yr-old female. The ovary including the mass measured 5 cm in size. On section, it revealed an unilocular cyst (4.5 cm in diameter) filled with mucinous fluid. There was a round, yellow, solid nodule, 1.5 cm in diameter within the wall. Microscopically, the cyst was lined by a single layer of endocervical mucinous epithelium and the nodule was composed of spindle cells showing an intersecting and whorled arrangement. There were cell nests showing polygonal shape with abundant cytoplasm among the spindle cells. They showed immunoreactivity for inhibin and did not have any connection with the adjacent mucinous epithelium. Therefore, we interpret the mucinous cystadenoma as having arisen de novo.