The efficient light energy usage in the chemical reaction system is an important issue in the field of photoelectrochemical region. The visible light induced hydrogen evolution reaction can be recognized as the one of the promising challenges. For the realization of it, the wide-band semiconductor electrode could be the good candidate. However, semiconductor electrodes often have the limitation to the ultraviolet region because of their wide band gap energy. As the breakthrough for it, the introduction of the plasmonic metal nanostructures has been received much attention because the system which is recognized as the plasmonic photoconversion electrode shows the visible light response characters. Up to now, although various systems have been achieved both the cathodic and anodic reactions by the selection of the materials, the molecular process especially on the plasmonic cathode system has not been fully understood yet. In our previous study, we have established the plasmonic photocathode by using the plasmonic nanostructure and the p-type semiconductor electrode [1]. In this system, the holes excited in the metal structures injected into the valence band of the p-type semiconductor and the remained electron are consumed in the hydrogen evolution reaction. It was interesting that, in the pH dependence examination, the photocurrent generations under the neutral condition was comparable to the acidic condition which is the favorable to the hydrogen evolution. This unique property could derive from the unique molecular process influenced by the plasmonic field excitation. In this study, we have attempted to investigate the kinetic isotope effect on the plasmonic cathode system for the clarification of the molecular process on it. Through the various photoelectrochemical measurements, we have confirmed the unique inverse isotope effect on the plasmonic cathode system under the specific condition. Our current research would provide the novel insight for the accurate design of the high efficient light conversion device. [1] H. Minamimoto et al., Chem. Lett., 2020, 49, 806.