Experimentally it was verified that the energy gap law differs greatly between the photoinduced charge separation (CS) and charge recombination (CR) reactions. As origins of its difference, we considered the non-linear response effect of solvent and the distribution effect of distances between donor and acceptor molecules. For this purposes, we first qualitatively investigated the role of non-linearity on the energy gap law by a phenomenological theory. Next, we developed a statistical mechanics theory for the ET rate. Based on the latter theory, we obtained a strict formula which holds between the energy gap laws of CS and CR reactions. Furthermore, it was theoretically proved that the inverted region in the CS reaction exists and its slope should be as steep as that of the normal region, so far as the electronic polarizability is not large. When the donor-acceptor distance is distributed, the energy gap laws are affected by means of three factors of electron tunneling matrix element, local energy gap and reorganization energy. By comparing numerically calculated results with the experimental data, it was clarified that most part of the origin of different energy gap laws between the CS and CR reactions is the distance distribution effect and some part is the non-linear response effect.