A 61-years old man with diabetes mellitus(DM) was admitted to our hospital, complaining of progressive right periorbital swelling, headache and toothache on the right lower second molar. On the first visit, moderate swelling was noticed from the right periorbital region with exophthalmos and subconjunctival effusion. Intraorally, right lower second molar had a severe periodontal disease and fistular formation on its distal area. From 3 days after hospitalization, the visual acuity of his right eye was gradually worsen and we performed CT scan. CT scan demonstrated an inflammatory change at the right orbit with subperiosteal abscess at the inferior orbital wall, which was extended from the right infratemporal, parapharyngeal and internal pterygoid space. Patient was treated by mean of intraoral(right upper vestibular and retromolar) and extraoral(infraorbital) incision and drainage, massive anti-therapy and DM control. The patient improved gradually and finally was discharged from the hospital, but his visual loss of right side was not recovered.