Optical second harmonic generation (SHG) spectra and SHG images were investigated on merocyanine dye (MD)/arachidie acid mixed monolayer at the air-water interface. A sharp peak of the SHG spectrum was observed at the J-band of 620nm. In the low wavelength region below 400nm, a sharp increase of the SHG spectrum was also observed. Careful comparisons of SHG images taken at several wavelengths revealed that the whole spectrum originates from the J-aggregate states of MD.