A simple analysis method for hypochlorous acid using a test strip and a smartphone has been developed. Hypochlorous acid is a component with a high bactericidal effect among bactericidal agents, however it is not possible to directly measure the colorimetricity of hypochlorous acid with commercially available test papers, and it is necessary to calculate the concentration from the results of colorimetric measurement of pH and total residual chlorine concentration. In this method, we have developed a simple analysis method for hypochlorous acid using a smartphone with original application software. The results show that the proposed method has sufficient sensitivity and accuracy as a simple method. Moreover, because of less susceptibility to interferences in the measurement environment, the hypochlorous acid can be simply quantified by holding the colorimetric plate and the color-plated test paper in front of the camera.