Our patient was a 37-year-old man with diabetes mellitus and hepatopathy as underlying diseases. Swelling, erythema and pain appeared in the left upper limb on the day before the initial examination. On examination, diffuse purpura was noted on the left upper limb, and, as it rapidly extended to the left upper trunk, emergency surgery was performed. Intraoperatively, gas-producing necrosis was observed not only in subcutaneous tissues but also from the fascia to muscle tissues, and the condition resembled clostridial gas gangrene. However, as the culturing of samples from the lesion yielded Bacillus cereus, a diagnosis of necrotizing fasciitis and myonecrosis (synergistic necrotizing cellulitis) due to B. cereus was made. While the patient developed a serious condition due to sepsis and disseminated intravascular coagulation, he could be saved by early debridement and intensive treatment with an appropriate selection of antibiotics.