A 72-year-old woman developed left-side ptosis and diplopia. Opthalmological examination showed left-side ptosis, disturbance of ocular movement in the upward and medial direction on the left side, upward diplopia and right-side gaze. These symptoms were exclusively due to left oculomotor nerve palsy. CT and MRI revealed that smooth-margined lesions in the left sphenoid sinus expanding laterally and compressing the left cavernous sinus. The patient was diagnosed with sphenoid sinus mucocele and underwent endonasal endoscopic surgery under general anesthesia. The sphenoid sinus mucocele was marsupialized widely via a trans-ethmoidal approach. White pus was observed in the lesion. About 4 months later, all symptoms had completely disappeared.