Patent ID: 6889017

Claim:
An image forming apparatus having a developing roller mounted to be spaced with a photo-sensitive body and supplying a developing agent to the photo-sensitive body, and a bias-applying part applying a predetermined bias to the developing roller through a current-conducting path to the photo-sensitive body from the developing roller, further comprising: a current detection part detecting a developing current flowing through the developing roller in response to the bias applied from the bias-applying part and generating a current value; and an engine control part controlling the bias-applying part to supply a first test AC voltage having a set first frequency to the developing roller and supply a second test AC voltage having a set second frequency to the developing roller, calculating a resistance of the developing roller and a developing gap between the developing roller and the photo-sensitive body in accordance with the current value detected from the current detection part respectively corresponding to the first and second AC voltages having the corresponding first and second frequencies, and controlling the bias-applying part to supply the developing roller with a bias voltage of a driving condition corresponding to the calculated resistance of the developing roller and the calculated developing gap.