To sterilize a sterilization filter unit with heated steam, minimize the heat load on the sterilization filter unit and extend the service life of the sterilization filter unit.
Heated steam is supplied to a sterilization filter unit that sterilizes supplied air, the temperature of the heated steam discharged from the sterilization filter unit is measured at predetermined time intervals, the F value is calculated from the measured temperature, and sterilization of the sterilization filter unit is ended when the F value reaches a target value.