The monolayer of amphiphilic merocyanine dye (MD) molecules at the air-water interface was observed by the grazing incidence X-ray diffraction (GIXD) method. At the air-water interface, the MD molecules formed two-dimensional crystallites (J-aggregates), whose excitonic absorption bands were located at longer wavelength than the monomer band. The structural analysis on the MD J-aggregate was carried out, and, using the determined structure, the relationship between the molecular arrangement and the energy shift of the visible absorption band upon crystallization was clarified. For the calculation of the energy shift, simple formulas were derived on the basis of molecular exciton theory. It was revealed that not only the transition dipole moment interaction but also the electric dipole moment interaction among the molecules plays an important role in the energy shift.