Photocurrent was generated by irradiation of a porphyrin self-assembly monolayer on a gold film electrode with neat-infrared (IR) as well as visible light under the surface plasmon (SP) resonance conditions. The action spectrum by the SP excitation demonstrated existence of discrete active bands in the near-IR region together with the Soret and Q-bands. These bands may be attributable to the absorption of the aggregates and the singlet-triplet transition of the porphyrin. The strongly enhanced electromagnetic field of SP in the near-IR is considered to undergo excitation of very weak discrete absorption bands which cannot be observed by the conventional absorption measurement.