The molecular alignment of a merocyanine (MC) J-aggregate monolayer at the air-water interface was determined by a grazing incidence x-ray diffraction method. The obtained molecular arrangement apparently shows that the conventional formula, which accounts only for the transition dipole interaction, is not sufficient to figure out the exciton band wavelength, suggesting the importance of the electric dipole (ED) interaction. We derived a simple formula for the ED interaction energy under an extended dipole approximation and clarified the ED contribution in the MC J aggregate.