A 15-year-old man was admitted because of diplopia and bilateral ptosis which occurred a few days after initial clinical signs, such as fever up, nausea, vomiting and headache. His pupils were anisocoric (Rt. phi 3.5 mm < Lt. phi 6.0 mm). In his left eye, light reflex was absent and its movements were limited in all directions. Brain MRI revealed the findings of paranasal sinusitis in bilateral ethmoidal and sphenoidal sinuses and swelling of bilateral cavernous sinus. Combination of intravenous antibiotic therapy and drainage improved his clinical symptoms and MRI findings. It was diagnosed as the inflammation originated in the sphenoid and ethmoid sinuses, which extended to the cavernous sinus and then involved III, IV, and VI cranial nerves. In conclusion, MRI was very useful to detect the cavernous sinusitis secondary to sphenoidal sinusitis.