The excitation of singlet reaction paths through a conical intersection for the [2 + 2] photocycloaddition of two ethylene molecules was studied by the complete active space self-consistent-field method. The electronic states in the conical intersection region were analyzed by the CiLC (CI/LMO/CASSCF) method. Drastic variations in the bond weights observed by CiLC analysis at the circle around the conical intersection corresponded with those observed by dynamic reaction coordinate calculations. The conical intersection was found to have a polarized conformation.