We observed a highly enhanced photoelectric conversion with a microscopically roughened Au electrode modified by Jand/or H-aggregate of cyanine dyes with an intermediate molecular spacer (a self-assembled monolayer of aminoalkanethiol) in between. In addition to the increased light absorption, as attributed to the increase in the amount of adsorbed dyes (specific surface area effect) and to a light scattering effect, the excitation of the surface plasmon plays an important role in causing further enhancement in the effective light absorption by the coupling between the surface plasmon and the dye transition dipoles.