An image forming apparatus includes an image bearing member, a transfer roller, an electric energy supply member, a voltage source, a detector, and a controller for adjusting a transfer voltage. A circumferential length 2K of the transfer roller and a distance k measured along an outer periphery of the transfer roller from the electric energy supplying member to the transfer roller satisfy 0.8K≤k≤1.2K. The controller controls the voltage source so as to satisfy 0.7T≤t≤1.3T, where 2T is time required for one full rotation of the transfer roller, and t is time during which the voltage source supplies the test current or the test voltage per one level in the operation of a setting mode.