PURPOSE: To efficiently inject a decoloring material by operating the optimum injection amt. from the integrated operation value of the light absorption curve of colored water and a parameter such as the max. absorption wavelength or flow rate of the colored water according to a predetermined operation formula and injecting the decoloring material into the colored water in the optimum injection amt.

CONSTITUTION: An operation means 1 considers the visible region of a person to set a wavelength range and calculates the light absorption curve of colored water to be decolored and uses the max. absorption wavelength, transmissivity, absorbancy and the relative difference between them as parameters to calculate the optimum injection amt. on the basis of the integrated operation value of the light absorption curve according to a predetermined operation formula. For example, since colored water is stable from the aspect of energy as the max. absorption wavelength becomes shorter even if the integrated operation value is same, it is necessary to allow a high concn. decoloring material to act. An injection amt. control means 2 controls the injection amt. of the decoloring material according to the calculated optimum injection amt. By this constitution, the decorloring material can be injected efficiently.