A case of gas gangrene is reported. A 66-year-old man was referred to our hospital because of swelling and dull pain in the right cheek region. He had been receiving treatment for diabetes mellitus, hypertension, and heart failure. Hemodialysis had been performed because of diabetic nephropathy. Antibiotics were given and surgical drainage was performed with the patient under local anesthesia on the first day of hospitalization. A microbiological examination revealed Streptococcus intermedius, Bacteroides species, and Fusobactelium sp. in the abscess. Despite treatment, the swelling and dull pain did not resolve. On the 7th day of hospitalization, a computed tomography examination revealed extended gas bubbles and an abscess in the soft tissues of the submandibular and cervical regions. Surgical drainage was performed under general anesthesia. Microbiological examination of the abscess revealed Gemella sp. and Provotella sp. The patient was discharged after 90 days of hospitalization. This report emphasizes the importance of rapid diagnosis of gas gangrene by computed tomography, followed bysurgical drainage, and the appropriate administration of antibiotics, especially in patients with diabetic nephropathy.