An image forming apparatus includes a photoreceptor, an exposure unit, a developing unit, and a control unit. The control unit sets the toner-layer-forming electric potential difference lower than an electric potential when forming the image on the print medium, forms a combined patch for calibration by the set toner-layer-forming electric potential difference for calibration, so as to adjust the developing-bias potential by measuring a print density of the formed combined patch. The combined patch includes a first patch and a plurality of second patches. The plurality of second patches have a width within a range of 0.2 mm to the predetermined value in the peripheral-velocity direction of the photoreceptor, extend with a length equal to or more than the predetermined value in the rotation-shaft direction of the photoreceptor, and are arranged at a predetermined interval in the peripheral-velocity direction of the photoreceptor.