A steam sterilizer includes a sterilization tank configured to house an object to be sterilized and perform sterilization on the object to be sterilized by maintaining supplied steam at a predetermined temperature and under a predetermined pressure; a steam-supplying pipe configured to supply the steam into the sterilization tank; a steam-supplying pipe valve configured to control an amount of steam flow flowing through the steam-supplying pipe, wherein the steam-supplying pipe is provided so as to have a diameter that allows a pressure-increase rate of 100 kPa/min or more in the sterilization tank; and a control unit configured to control the steam-supplying pipe valve to be fully opened, in a conditioning process for repeatedly performing introduction and discharge of the steam in and from the sterilization tank and to control the steam-supplying pipe valve so as to supply a predetermined amount of steam, in a sterilization process after the conditioning process.