A case of successful pregnancy after treatment of stage Ia malignant mixed Müllerian tumor of the ovary is described. This tumor is very rare and usually occurs in postmenopausal women. cis-platinum-based chemotherapy following the primary operation in the early stage is the most effective treatment, although most of the tumors have been found in the advanced stage. In this patient, treatment with unilateral salpingo-oophorectomy to preserve fertility was performed and followed by cis-platinum-based chemotherapy for 5 years. Two years after completion of the chemotherapy, she spontaneously conceived and the course of the pregnancy has been uneventful to this point.