Microscopic contact-electrified charges on a silicon oxide layer formed on a p-type Si substrate were investigated in air using an atomic force microscope with a biased conductive cantilever. We could successfully image the dissipation of the contact-electrified charges after a single contact. We found that the sign of the contact-electrified charges is reproducible and controllable by the polarity of the bias voltage. We also found that negative charges are more easily deposited on the silicon oxide surface than the positive charges. Furthermore, we found that the charge dissipation as a function of the time after contact electrification takes two steps for the negative charges. At the first step the charge dissipation is nearly same for both the repulsive and the attractive force measurements, while at the second step the dissipation for the attractive force measurement is much faster than that for the repulsive force measurement.