Provided is a steam sterilizer capable of performing water vapor sterilization without using a vacuum pump as a mechanism for bringing the inside of a pressure vessel into vacuum. The steam sterilizer includes a pressure vessel (32) which stores an object to be sterilized and in which sterilization is performed with water vapor supplied from a steam piping, and vacuum-generating means (34) which is connected to the pressure vessel (32) and brings the inside of the pressure vessel (32) into a vacuum state. The vacuum-generating means (34) includes a water ejector (48), a tank (50) for storing supply water supplied from an outside, and a pump (52) for supplying the supply water inside the tank (50) to the water ejector (48).
