An osteoma is slowly growing benign tumor of bone, which occasionally produce symptoms of space-occupying lesion. Osteoma of the paranasal sinus is the most common benign tumor originated from the paransal sinus and the frontal sinus is the most frequent site. Its intracranial extension is rare. Recently the authors had experienced one case of large osteoma in the frontal sinus with intracranial extension. The tumor was removed successively by bicoronal osteoplastic frontal sinus operation.