The method for producing dinitrogen tetraoxide comprising the steps of operating a decompressor (3) to reduce a pressure within the space area (2) maintained in the hermetically sealed condition to the decompressed or vacuum condition below the saturated vapor pressure curve of dinitrogen tetraoxide, supplying the dinitrogen tetraoxide liquid as a sterilant feedstock from a vessel (4) to a spray device (25) while preventing gasification of the dinitrogen tetraoxide liquid on the way between the vessel (4) and spray device (25), and spraying the dinitrogen tetraoxide liquid through the spray device (25) into the space area (2) under the decompressed or vacuum condition to, at the time of the spraying, gasify at least some of the dinitrogen tetraoxide liquid within the space area (2) decompressed below the saturated vapor pressure curve.
