We report a patient with a necrotizing fasciitis whose life could be saved by amputation of the upper limb. A 57-year-old man had watery diarrhea and right ring finger pain without a definite trauma. He visited other hospital. But two days later, he had severe pain, swelling, and blood circulation loss at the finger. The patient was therefore transferred to our hospital. Within a few hours, skin lesion aggravated, and spread to his right shoulder and around the trunk. Necrotizing fasciitis was diagnosed and his upper right limb was amputated. Streptococcus pyogenes was cultured in his blood. Intravenous antibiotics, including penicillin G and clindamycin, were added just after operation. About two months after operation, his general conditions improved.