Proximity effects of charge groups deposited by contact electrification on thin silicon oxide in air were investigated using a modified atomic force microscope. For initial separations L of L similar to 3.0 mu m, oppositely charged groups recede from each other in distance with time after contact electrification. On the other hand, a couple of negative (unipolar) charge groups approach each other with time. These features are contrary to the expected approach or recession due to the Coulomb attractive or repulsive forcer respectively. Furthermore, the critical time t(c) of a stable-unstable phase transition of negative charges becomes shorter or longer due to the proximity effects by the opposite or same charge groups, respectively. These proximity effects seem to be induced by the interplay of the Coulomb force and the surface charge diffusion.