We report a successful spontaneous pregnancy in a patient who underwent ovarian transposition and radiotherapy after the diagnosis of giant-cell tumor at 13 years of age. She had a tumor on the sacral bone extending to the whole second sacrum anteriorly, posteriorly, and both laterally and bulging out to the neural canal. She underwent debulking surgery twice; no pelvic organs, including the uterus, ovaries, and bowel, were affected by the giant-cell tumor. Moreover, ovarian transposition following debulking surgeries was performed, followed by radiotherapy in the same year. After 20 years (at 34 years of age), she spontaneously conceived and delivered a healthy baby at 38.2 weeks of gestation without obstetrical complications. In case of giant-cell tumor with sarcomatous transformation, radiotherapy is recommended after surgical removal. Therefore, fertility-sparing procedures, such as ovarian transposition, are very important in patients who want to conceive, and spontaneous pregnancy is still possible in such patients.