Abstract: A sterilizer using steam, which can assure maintaining the temperature at the inside of a thing to be sterilized at a sterilizing temperature during a predetermined period, is proposed. The time to maintain the steam temperature at boiling, temperature is determined as a function of the time which was necessary to raise the steam temperature from a predetermined starting temperature to a predetermined end temperature. After maintaining the boiling temperature, a valve is closed to raise the steam temperature to the sterilizing temperature, which sterilizing temperature is maintained during a predetermined time period.