Two extreme cases can be solved for electron charging effects where the first one ignores the electron correlation and the second one treats only one-electron scattering. So far, Coulomb blockade has been treated by the orthodox theory [Likharev, IBM J. Res. Dev. 32, 144 (1988)], where the charging energy is expressed as e(2)/2C using the capacitance C of a tunnel junction. On the other hand, one-electron scattering can be solved exactly by using the Schrodinger equation. But it is difficult to solve for a many-body system by the Schrodinger equation. In this article, stress is placed on the case where the electric field cannot be ignored anymore. A treatment of the charging energy is proposed using a screening model or an electromagnetic field model by employing a cavity model in the field of photon.