It is an object to establish a sterilization condition and means for decreasing the amount of a remaining sterilization agent as much as possible after sterilizing a rubbery sealing tool. In a sterilizing method using gaseous hydrogen peroxide, a filling rate of a sample in a sterilization bag for a normal type is set to 45% or more and a sterilization condition is set to a hydrogen peroxide treatment of 3 pulses and an aeration pulse of 20 pulses, thereby carrying out a sterilization treatment. Furthermore, it is preferable that a volume rate in a bag of an outer bag accommodating the sterilization bag should be set to 55 to 12%. By such setting, a survival ratio of 0/10 of a bacteria cell could be realized at first to fifth times of repetition of the sterilization treatment.