It is considered that benign tumors of the nasal and paranasal sinuses are relatively rare tumors, especially they occur very rarely in the maxillary sinus. Recently, the author experienced a rather rare case of fibro-osteoma developed in the maxillary sinus which caused exophthalmos of the left eye. This 23 years old male was admitted to EYE section of our hospital complainig of diplopia, epiphora, progressive exophthalmos of left eye and upper deviation of the left eye ball for about several months duration. He was consulted to ENT section because of nasal obstruction and nasal discharges, and immediate x-ray examination was taken. It was suspected as a mass in the left maxillary sinus with the x-ray findings, and biopsy was performed with Caldwell-Lucs approach. Under the confirmative diagnosis of fibro-osteoma of the left maxillary sinus with the histopathological study, complete removal of the tumor mass was tried successfully with partial maxillectomy. Post operative patient's condition was uneventful with no evidence of recurrence.