In the conventional types of Active Transfer Voltage Control (ATVC) system, it is assumed that transferring material has definite impedance. When a toner image is transferred to the transferring material which is in a condition where only the top of the transferring material has high impedance, the conventional ATVC system maintains a transferring current higher than the critical transferring current value with the top of the transferring material and applies a bias voltage at a corrected level, whereby the ATVC system may supply an excessive current to a portion of the photosensitive drum corresponding to a location which is other than the top having the high impedance and at which a toner image is not formed. Therefore, it is provided, an image forming apparatus which is capable of favorably transferring a toner image regardless of impedance of a transferring material.