An image forming apparatus for overlaying toner images of at least two colors on an image carrier, and forming the toner images on a printing medium, counts the perimeter of the image carrier on the basis of a reference signal and a sampling period of a clock count, and sets target values of image formation start timings for respective colors in consideration of a perimeter error of the image carrier due to a quantization error produced upon counting on the basis of the reference signal and the sampling period of the clock count.