The manganese cluster (CaMn4O5) in the photo system II (PSII) is the reaction center of the light-driven oxidation reaction, which generates the molecular oxygen. In this paper, we address the issue of the effect of the environment on the free energy associated with the oxidation of the Mn cluster in S-1 state by conducting the large-scale quantum mechanical/molecular mechanical simulations, which involve the whole of the PSII monomer. It was found by the simulations at the level of the B3LYP functional that the environment surrounding the Mn cluster reduces the vertical oxidation free energy Delta mu(vrt), by 64.8 kcal/mol. A decomposition analysis of the free energy Delta mu(vrt) revealed that the system composed of peptide chains, ligands, lipids, and potassium ions contributes to lowering of Delta mu(vrt) by -98.0 kcal/mol, whereas the solvent water makes an opposite contribution of 38.9 kcal/mol. Reduction of the vertical oxidation free energy directly leads to the lowering of the activation free energy Delta G(ac), for the electron transfer reaction from the Mn cluster in S-1 state to the neighboring Tyr(z)(+). Consequently, the electron transfer rate was found to be enhanced by a factor of 10(12) by virtue of the influence of the environment.