A new method to evaluate the weights of resonance structures from molecular orbital wave function is proposed, which is based on the second quantization of singlet-coupling. The present method is useful to analyze molecules of which the electronic structures are well localizable. The evaluation is carried out through localization of molecular orbitals followed by algebraic calculation of density matrices. This method is applied to H2O, H3O+, and BH3. The calculated weights of covalent and ionic structures are in excellent agreement with those of the previous works and our chemical intuition.