PROBLEM TO BE SOLVED: To provide a steam sterilizer for aiming to reduce the temperature in a machine room and its adjacent room and reducing an equipment cost and energy cost devised by countermeasure against high temperature.SOLUTION: A steam sterilizer is used for sterilization operation including a warming-up operation for warming a sterilization room by supplying steam to a steam jacket 2 during starting and to the sterilization room 1 if necessary, a preheating process for warming a sterilized subject stored in the sterilization room by heat from the steam jacket, an air emission process for emitting the air in the sterilization room, a steam supplying process for supplying steam to the saturated vapor pressure at the preset temperature in the sterilization room, a sterilization process for holding the preset temperature in the sterilization room during the predetermined period, a steam emission process for emitting the steam in the sterilization room after sterilization, and a dry process for drying the sterilized subject in the sterilization room, and it has a control means for closing a steam supply valve 3 of an outer can which supplies the steam to the steam jacket and blocking the steam supply to the steam jacket after the end of the warming-up operation before the start of a first sterilization operation and before the start of the next sterilization operation after the end of sterilization operation.
