An image forming apparatus includes plural image forming sections for forming toner images of different colors. A patch toner image forming section forms patch toner images side-by-side onto a moving transfer body by using the image forming sections. A spread clock signal output section spreads a clock signal having a predetermined standard frequency at a predetermined spreading ratio and outputs the clock signal. A patch toner image intervals calculating section calculates intervals between the patch toner images on the transfer body based on the number of clock signals outputted from the spread clock signal output section and the standard frequency. A calculation result correcting section counts spread clock signals outputted during a time interval from the spread clock signal output section, and corrects a calculation result given by the patch toner image intervals calculating section based on the result of the counting.