The present invention provides medicines for preventing or treating infectious diseases, sepsis and/or septic shock, which exhibit excellent immunostimulating effects. More specifically, the invention provides medicines for immunostimulation and infection-protection and -treatment, and/or for preventing or treating sepsis and septic shock, which comprise reduced riboflavin and/or reduced riboflavin derivative or pharmacologically acceptable salt thereof. Also provided are methods for using them in the prevention and/or treatment of infectious disorders such as sepsis and septic shock.