  A new approach of the QCD sum rule is proposed in which positive and
negative-parity baryons couple with each other. With positive and
negative-parity states explicitly taken into account, sum rules are derived by
means of the dispersion relation in energy. The method is applied to the
nucleon channel and the parity splitting of the nucleon resonance states is
studied. It is found that the obtained sum rules have a very good Borel
stability. This suggests that the ansatz for the spectral function in the
present sum rule approximates the physical spectrum better than the usual
lowest pole plus continuum ansatz. The predicted masses of the positive and
negative nucleons reproduce the experimental ones fairly well. Especially, the
mass difference is extremely close to the experimental value.
