We have found, for the first time, a unique spectroscopic property of noble metal nano-particles (colloidal silver in solution or noble metal island film) covered with a J-aggregate monolayer. The spectral dip or hole at the position of the J-band was observed in the surface plasmon band. The appearance of the hole was related to the interaction between the surface plasmon of the noble metal nano-particles and the Frenkel exciton of the J-aggregate.