When retroperitoneal leiomyosarcoma develops in pelvic cavity, it often presents similar symptoms and radiological findings to adnexal tumor, therefore obscures diagnostic approaches until an exploratory laparotomy is performed. We report an unusual presentation of retroperitoneal leiomyosarcoma mimicking an adnexal tumor with extremely elevated serum CA-19-9. Though the most of the prominent mass was removed during surgery, there was massive bleeding due to tearing of internal iliac vein while dissecting the ureter close to vessels. This case focuses on the significance of considering retroperitoneal tumor even if the mass is located in ovarian fossa and has highly elevated serum level of CA-19-9. And in attempt of tumor removal, the excision needs to be clean-cut without damaging nerves or vessels around the mass and avoid causing any prospective complications.