A 68-year-old woman presented a one-month history of lower abdominal pain and weight loss, and was admitted to our hospital. On physical examination, a large hard mass was palpated in her right lower abdomen. An ultrasonograph and computed tomographic (CT) scan revealed a right ovarian tumor that measured 6.9 x 4.9 cm in size. A total hysterectomy and bilateral salpingo-oophorectomy were performed. The postoperative diagnosis of the tumor was squamous cell carcinoma (SCC) of the ovary. She died of infection and disseminated intravascular coagulation 5 months after surgery. The clinical and autopsy examinations did not show the primary lesions of SCC except in the right ovary. Mature cystic teratoma, Brenner tumor and endometriosis, which are ordinary regarded as the histogenesis of ovarian SCC, were not found, but a few surface epithelial inclusion cysts with squamous metaplasia were observed in non-cancerous area of the right ovary, and the contiguous transition from the metaplastic cyst wall to SCC was confirmed by stepwise serial sections. The present case suggests that the surface epithelium of ovary could be the fourth possibility in the histogenesis of the ovarian SCC.