Osteoma in the maxillary sinus, especially isolated osteoma, is very rare in Japan. We present a case of isolated osteoma in the maxillary sinus that was detected by chance on cephalograms obtained before surgical orthodontic treatment to correct facial asymmetry. There were no signs or symptoms caused by the osteoma. After orthodontic treatment for 3 years, the tumor was removed simultaneously with Le Fort I osteotomy under general anesthesia. The tumor was encapsulated by fibrous tissue and was isolated from the maxillary sinus wall. It measured 28×14×12 mm. Histopathologically, the lesion was diagnosed as an osteoma. There has been no sign of recurrence for 3 years after surgery. Reports of Japanese cases of osteomas of the maxillary sinus are clinicopathologically reviewed.