A photoreceptor cleanerless image forming apparatus and method for overlappingly forming yellow, cyan, magenta, and black toner images and decreasing color mixture or an exposure error due to reverse transfer toner or untransferred toner are provided. The image forming apparatus 100 comprises four image forming units 100a, 100b, 100c, and 100d configured to be photoreceptor cleanerless in a 4-drum tandem manner. Each image forming unit includes a photoreceptor 103a, 103b, 103c, or 103d, a charger 105a, 105b, 105c, or 105d, an exposure apparatus 106a, 106b, 106c, or 106d, and a developing apparatus 109a, 109b, 109c, or 109d. When exposure resolutions Ry, Rc, Rm, and Rk are assumed for exposure sources of the exposure apparatuses in the image forming units which form yellow, cyan, magenta, and black images, respectively, the exposure resolutions are configured to satisfy the conditions of Rk≦Rc≦Rm and Rm>Rk. This decreases an exposure error (image hysteresis) in an image formed on paper.