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 ase2/2Cusing the capacitanceCof a tunnel junction. On the other hand, one‐electron scattering can be solved exactly by using the Schrödinger equation. But it is difficult to solve for a many‐body system by the Schrödinger 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.