An accurate discrimination system of the color-change point for detector tubes has been developed. A gaseous or liquid sample is passed through a tube that packs granules coated with a colorimetric reagent, and the color-change point is discriminated by the developed system. The tube is photographed by a digital still camera, and the image is analyzed by color-analysis software installed on a PC. This software digitizes the color information of each point under the line specified by the user, and displays a graph. In order to reduce noise on the graph, the moving average, as well as fast-Fourier transformation and inverse fast-Fourier transformation smoothing functions are implemented. A differentiation function used to identify the color-change point and color-control function to emphasize the color change are also implemented. This system has been applied to detector tubes including ammonia, hydrogen chloride, chloride anion, toluene, water vapor, and 2-propanol, and their color-change points have been successfully discriminated.