Retinoid molecules are the important component for photoreaction. We observed absorption spectra of the solution of retinoic acid, soaked films, and LB films. The spectrum of the solution shows the peak at 368nm. The spectrum of the soaked film could be resolved into two peaks. The peak at 370nm would be due to the monomer molecules, and the peak at 418nm would be due to the aggregate. The absorption spectrum of the LB film of retinoic acid shows the main peak at 380nm (M), subpeak at 358nm (A2) and the shoulder peak at 420nm (A1). The main peak (M) is considered to be the monomer molecules. Eight kinds of aggregates of retinoic acid molecules were considered, and the shifts of absorption transition energies of the aggregates were calculated using the semiempirical electronic structure program. The results suggest that the peak A2 at 358nm should be due to the trimer of retinoic acid.