Metal nanostructures have the potential to increase photon-molecule interaction efficiency due to their plasmonic antenna effect. However, plasmonic enhancement of the efficiency of photochemical reactions such as photosynthesis is rather difficult because excited molecules are easily quenched near the metal surface. Sphere-plane metal nanostructures have an advantage to reduce such undesirable contribution due to their controllability of metal-molecule interfaces. Here, structural tuning of optical antenna properties, which is indispensable for selective enhancement of photochemical reactions, is demonstrated as plasmonic enhancement of photoinduced electron transfer reactions on porphyrin-based monolayer systems using the sphere-plane nanoantennas. The efficiency of photocurrent generation was substantially enhanced in good agreement with the plasmonic resonance properties of the optical antenna. The contribution of the excited state quenching was also evaluated by comparing the enhancement factor of the photocurrent and the degree of the field enhancement. This result provides useful insights for designing efficient plasmonic photochemical systems.