PROBLEM TO BE SOLVED: To efficiently disinfect an inside of an aseptic filling chamber.SOLUTION: Provided is a sterilization method of an inside of an aseptic filling chamber in which at least hydrogen peroxide water is injected in an inside of an aseptic filling chamber (11) in which a filler (12) is housed, thereby an inside of the aseptic filling chamber (11) undergoes disinfection treatment, wherein hydrogen peroxide water is injected to the filler (12) synchronizing with SIP that is performed by high temperature moisture vapor. Bactericidal effect by injection of hydrogen peroxide water can be further raised using heat of moisture vapor at SIP heat-transferred to an inside of the aseptic filling chamber (11).