A 59-year-old woman was referred to our clinic with sudden visual loss in her right eye after she was treated with 40 mg/day of oral prednisolone for 2 weeks under the diagnosis of idiopathic optic neuritis. At that time, computerized tomography (CT) of the brain showed no evidence of optic nerve or brain pathology. However, there was progressive diminution of right visual acuity associated with a limitation of adduction and abduction in the right eye. On magnetic resonance imaging and repeated CT, a malignant lesion was suggested, and was confirmed as an Aspergillus fungus colony by histopathologic examination. Postoperatively, she was treated with intravenous administration of amphotericin B for 13 weeks. However, her condition continued to deteriorate. She developed ptosis and total ophthalmoplegia in the right eye and blindness in both eyes. After discharge, she was given itraconazole for 20 weeks. She has shown no recovery of visual acuity or extraocular motion during a two-year follow-up period. The clinical features of our case suggest that early diagnosis in a case of aspergilloma presenting with visual loss is difficult and that a high index of suspicion, repeated radiological examination and adequate biopsy may be required for diagnosis.