Gas gangrene is a severe bacterial infection accompanied by gas production by either clostridium genus or non-clostridium genus. A 76-year-old man visited our hospital with complaints of left buccal swelling and trismus. CT examination revealed periapical lesions in the left lower molars and abscess formation with gas retention, extending from the left submandibular space to the temporal fossa. Drainage of the abscess was immediately performed by submandibular, temporal and intraoral incisions, with a concomitant administration of anti-microbial agents. Leukocytosis and increase of body temperature indicated that the patient also was accompanied by systemic inflammatory response syndrome (SIRS). During treatment of the gas gangrene, a diagnosis of dementia and diabetes mellitus was made. Although the dementia and diabetes mellitus delayed his recovery from the infection, he left the ICU 34 days after surgical intervention.