A 52-year-old man with type 2 diabetes mellitus was treated for 14 yeas after diagnosis. Insulin therapy was started 5 years ago and glycemic control was good, with no evident diabetic complication. He reported general malaise and laboratory data showed hyperglycemia. A physical examination revealed swelling of the scrotum and penis, skin redness of the abdominal wall and right hip regions, and a foul smell. He was diagnosed as having gas-producing Fournier's gangrene based on the findings of crepitation by palpation on the abdominal wall and spotty gas images in the scrotum by X-ray examination. He immediately underwent extensive debridement of necrotic tissues and autoskin transplantation. A fish bone 2 cm long was found in the right ischiorectal fossa during surgery, indicating the trigger of infection. Bacteroides fragilis was isolated from blood culture and E. coil and C.L. perfringens from pus in the scrotum. He received hyperbaric oxygenation therapy and antibiotics for sepsis, anticoagulants for disseminated intravascular coagulation, continuous hemodiafiltration for acute renal failure, and extensive autoskin transplantation. After again undergoing debridement for widely spread skin redness on day 4, his general conditon improved and he was discharged on day 88. This case study describes the therapy used to treat a 52-year-old patient with severe gas-producing Fournier's gangrene.