A necrotizing fasciitis especially caused by group A streptococcal infection is a life-threatening disease. This infection cause death due to septic shock and multiple organ failure in a short time without the immediate and adequate treatment. Currently a rapid test kit for streptococcal pharyngitis (strep A) is useful for prediction of group A streptococcal infection. We here demonstrate a 61 years old man's case of life-saved necrotizing fasciitis in genital area (Fournier's gangrene) by group A streptococcus infection, and usefulness of this kit for rapid diagnosis, aggressive debridement, and selection of adequate antibiotics.