Ovarian endometriosis sometimes develops into ovarian cancer, especially clear cell adenocarcinoma and endometrioid adenocarcinoma. However, endometriosis rarely develops into squamous cell carcinoma. We present a case of squamous cell carcinoma arising from endometriosis. A 47-year-old Japanese woman was given a diagnosis of ovarian squamous cell carcinoma arising from endometriosis. She was treated with combination chemotherapy consisting of paclitaxel and carboplatin once every three weeks. Four months after the initial chemotherapy, multiple liver tumors appeared, and her treatment was changed to palliative therapy. Based on this case, in which ovarian squamous cell carcinoma arose from endometriosis, endometriosis should be followed-up strictly.