The optomotor response (OMR) is a locomotor behavior of animals that is induced by moving repetitive visual stimuli. This characteristic helps animals particularly when stabilizing and maintaining position in schools and herds. Here, we developed a simple but sensitive method for quantifying the OMR using medaka (Oryzias latipes) as a model. This method, which simply requires video-recorded behavior, free tracking software, and a generic spreadsheet program, enables the evaluation of spectral sensitivity by researchers with little knowledge about the behavioral characteristics of the test animal or of the OMR. Based on a manual method, we reported previously that wild-type and red-colorblind medaka exhibited an OMR up to λ=830 and 740 nm, respectively. However, the present method, which quantifies the OMR according to three parameters (starting time, duration, and total distance of swimming) that are calculated based on a series of x–y coordinates of the moving fish, supported that conclusion and further indicated that both strains perceive light at even longer wavelengths. This low-cost, quantitative, and semi-automatic method would widen the opportunities to unveil behavioral photosensitivity in animals of interest.