To provide a steam sterilizer that can display given performance by means of a miniaturized water sealing type vacuum pump.

This sterilizer is equipped with a pressure vessel 10 and vacuum generating means. The pressure vessel 10 sterilizes inserted objects for sterilization by means of steam that is supplied from the steam supply piping. The vacuum generating means generates a vacuum state in the pressure vessel 10. A water sealing type vacuum pump VP is used as the vacuum generating means, in which an impeller eccentrically mounted inside the easing in which water is stored is rotated by driving means for intake and exhaust of air containing steam and droplets. Furthermore, on the piping from the steam exhaust port in the pressure vessel 10 to the water sealing type vacuum pump VP, a water spray device 46 is disposed for spraying water to the exhaust gas containing steam and droplets that is exhausted from the pressure vessel 10.