Abstract: After making the relative humidity in a working chamber 1 to 10% or less beforehand, hydrogen peroxide vapor supply means 2 is operated, and hydrogen peroxide solution is dropped from an injector 14 into an evaporator 12 for 3 minutes at a flow rate of 5 g/min. Thereby, hydrogen peroxide vapor is rapidly supplied to the above-mentioned working chamber 1, and the concentration of the hydrogen peroxide vapor in the working chamber 1 is set to about 1300 ppm. Dropping of hydrogen peroxide solution with the injector 14 is stopped and the state of a closed circuit between the working chamber and the hydrogen peroxide vapor supply means is maintained for a predetermined time, and if the concentration of hydrogen peroxide vapor becomes the predetermined concentration or below, ventilation means 4 is operated to remove the hydrogen peroxide vapor in the working chamber, and sterilization is terminated.