A real-time measuring method of a dispersive dye, i.e. oil dye, has been developed which enables us to measure the concentration and its fluctuation of the three base colours. The method is based on the light scattering theory and its principle requires three different lasers having the three base colours. The system consisted of a semiconductor laser and argon ion laser for light sources, a photodiode for a light detector and a personal computer for data processing. Detection sensitivity for the dye concentration was about 1 mg I l and was applicable for practical use.