To provide a selecting method of raw material solution supply quantity at the time of producing an aq. chlorine dioxide obtained with high quality at a high yield and hardly containing by-produced chlorine, the producing method of the aq. chlorine dioxide, the producing device and a measuring device of a chlorine dioxide concentration.

At the time of continuously producing the aq. chlorine dioxide from chlorous acid, hypochlorous acid and an acid, the supply flow rate of each raw material solution is selected. By keeping the supply flow rate of the chlorous acid solution constant and changing the supply flow rate of the acid solution the supply flows rate of the acid solution is searched so that the chlorine dioxide concentration in the chlorine dioxide aq. solution is maximal and simultaneously the pH of the chlorine dioxide aq. solution is 2.0-6.5. Next, by changing the supply flow rate of the hypochlorous acid solution, the supply flow rate of hypochlorous acid solution is searched so that the chlorine dioxide concentration is max. and simultaneously the pH of the chlorine aq. solution is 2.0-6.5. The supply quantity of the searched acid solution and the hypochlorous acid solution and the supply quantity of the chlorite kept constant are selected.