An apparent large absorption reduction was observed for a thiacyanine J aggregate in solution. Since fluorescence microscopy proved that the J aggregate formed a mesoscopic rod-like structure (several tens of micrometers in length and about 1 mum in width) in solution, the reduction was due to that of the apparent absorption cross-section caused by formation of the aggregate particles, indicating that no light transmitted through the region of particles composed of strongly absorbing chromophore units with the spectroscopic characteristics of J aggregates. The absorption behaviors were also simulated quantitatively by the present model.