An image forming apparatus includes an image forming unit that forms a developer image, a transfer portion that transfers the developer image to a transfer object, an endless transfer belt that rotates passing between the image forming unit and the transfer portion, a driving roller that drives the transfer belt to rotate, a plurality of marks provided on the transfer belt and arranged in a rotating direction of the transfer belt, a measurement unit that measures a rotation period of the transfer belt using the plurality of marks, and a controller that adjusts a timing at which the image forming unit forms the developer image, based on a measurement result of the rotation period measured by the measurement unit. A circumferential length L of the transfer belt in the rotating direction has a length based on a natural number multiple of a circumferential length m of the driving roller.