An object to be sterilized is disposed inside a vacuum chamber, and the inside of the vacuum chamber is evacuated. When the pressure inside the vacuum chamber reaches 3 Pa, a cutoff valve disposed between the vacuum chamber and the vacuum pump is closed. High-frequency power is supplied to an electrode. Oxygen gas is introduced into the vacuum chamber for 0.1 sec. Thereafter, the pressure is held constant for 3 sec. Until the pressure inside the vacuum chamber reaches 10 kPa, introduction of oxygen gas into the vacuum chamber is repeated. When the pressure reaches 10 kPa, the cutoff valve is opened to evacuate the inside of the vacuum chamber and the supply of the high-frequency power is stopped. Introducing the oxygen gas and determining pressure inside the vacuum chamber is repeated, and after 90 min, the pressure inside the vacuum chamber is returned to atmospheric pressure.