PROBLEM TO BE SOLVED: To eliminate the need of an expensive cylinder of diluted chlorine gas with the concentration adjusted even when a target space to be sterilized and decontaminated with chlorine dioxide gas is a large space of 1000 mor greater.SOLUTION: Compressed pure chlorine gas from a liquefied chlorine cylinder 2 and compressed nitrogen gas from a nitrogen gas cylinder 3 are adjusted to specified pressures by flow rate adjustment valves 12 and 22 and mixed to form a diluted chlorine gas of a specified concentration, and the diluted chlorine gas is introduced through a supply tube 31 into a filling container 41 filled with a granular solid chlorite to generate chlorine dioxide gas. When the ratio of the flow rates of the pure chlorine gas and a diluent gas exceeds predetermined upper limits of the concentrations of pure chlorine gas and the diluent gas on the basis of measurements of the flow meters 14 and 24, an on-off valve 15 for the pure chlorine gas is closed by a controller C.
