Abstract: Carbon dioxide is supplied from a carbon dioxide cylinder (15) to a pressure vessel (13) through reducing valves (18 and 19). The pressure vessel (13) is also supplied with a sodium hypochlorite water solution having a desired concentration through a material supply pipe (12). The material supply pipe (12) is connected to first and second branch pipes (100 and 101) through a distribution valve (102). The sodium hypochlorite water solution supplied through the material supply pipe (12) is partially sprinkled to a gas-phase region in the pressure vessel (13) through the first branch pipe 100, while the remainder of the sodium hypochlorite water solution is supplied to a liquid-phase region in the pressure vessel (13) through the second branch pipe (101). The pressure vessel (13) is provided with a liquid level retention mechanism (25 to 29) is provided to retain the liquid level in the pressure vessel (13) within a constant range.