PROBLEM TO BE SOLVED: To prevent water-hammer from occurring in a sterilizer in which a sterilizing process of supplying a heat exchanger with steam is carried out and subsequently a cooling process of supplying the heat exchanger with cooling water is carried out.SOLUTION: A treatment tank 3 houses a material 2 to be treated and stores water. A circulation means 9 circulates water stored in the treatment tank 3 between the treatment tank 3 and a heat exchanger 8. A steam supplying means 10 supplies the heat exchanger 8 with steam to heat circulation water circulated by the circulation means 9. A cooling means 11 supplies the heat exchanger 8 with cooling water to cool the circulation water circulated by the circulation means 9. A steam discharging means 13 discharges steam from the heat exchanger 8 and pipelines 29, 34 subsequent to stopping supplying the heat exchanger 8 with steam by the steam supplying means 10, and prior to starting supplying the heat exchanger 8 with cooling water by the cooling means 11. Since the cooling means 11 supplies the heat exchanger 8 with cooling water subsequent to discharging steam from the heat exchanger 8 by the steam discharging means 13, water-hammer can be prevented from occurring.
