Sphenoid sinus aspergillosis is notorious for its serious complications, such as permanent cranial nerve deficits and possible death. The most common associated symptoms are headache, followed by visual changes, and cranial nerve palsies. Because of an insidious onset, frequently resulting in missed and delayed diagnosis, sphenoid sinus aspergillosis is a potentially lethal medical condition. We report a case of visual loss secondary to isolated sphenoid sinus aspergillosis. A 69-year-old man presented to our hospital with the complaint of headache. The headache started one year previously and was described as severe dull pain localized bilaterally to the temporo-orbital region. The patient took daily NSAIDs for the pain. The neurological examination was normal. The MRI of the brain showed a left sphenoid sinusitis. A transnasal endoscopic superior meatal sphenoidotomy was performed. Aspergillosis was confirmed after a surgical biopsy was obtained. The patient was discharged from hospital without antifungal therapy. One month later, the patient complained of headache and loss of vision bilaterally. The orbital MRI showed a left cavernous sinus and bilateral optic nerve invasion. The loss of visions was permanent. In our case, the diagnosis was delayed; antifungal agents were not administered after surgery and the patient lost his vision as a result. Therefore, early diagnosis and proper treatment are important. Although the treatment of an invasive type of aspergillus has not been established, surgical removal of a nidus and aggressive antifungal therapy are recommended.