By focusing on the fact that nitrogen dioxide exhibits an increased sterilizing effect among other sterilant gases including nitrogen oxide, the present invention is made to provide a sterilization method which may be suitably used for sterilizing items to be sterilized such as medical instruments which require increased reliability by using a high concentration NO2 gas of 5,000 ppm or above, for example. An inside of a sterilizing chamber containing an item to be sterilized is humidified, and a concentration of NO2 in the sterilizing chamber is made to be from 9 to 100 mg/L by filling a high concentration NO2 gas.
