The sterilization system includes a plurality of spaces to be sterilized by a sterilization process; a gas supply source generating nitrogen dioxide gas; a piping system and a valve device; a nitrogen dioxide gas sensor; and a control device. The control device performs a gas filling process in which each of the plurality of spaces to be sterilized is connected to the gas supply source one by one for a first predetermined length of time and successively filled with the nitrogen dioxide gas by controlling the valve device, and when the concentration of the nitrogen dioxide gas in a space to be sterilized that is being filled with gas is detected by the sensor to have reached a predetermined level, the control device performs control such that the subsequent gas filling process is performed with this space to be sterilized being excluded from the gas filling process.