The influence of substance transport (diffusion resistance) of the reactants should be eliminated in order to extract catalytic activity from the reaction rate of an electrochemical reaction. Diffusion resistance is proportional to the “thickness of the diffusion layer” on the electrode surface. RDE can control the “thickness of the diffusion layer” on the electrode surface by the electrode rotational speed. By acquiring the sensitivity of the reaction rate to the rotational speed, the diffusion resistance can be eliminated and the charge transfer rate (activity) on the electrode surface can be analyzed.