An image forming apparatus, includes a plurality of image bearing members, respectively provided with image bearing member pulleys coaxially for rotating in one body with the corresponding image bearing members; an idler pulley, disposed between the image bearing members; a single drive belt, wound over the image bearing member pulleys for rotatively driving the image bearing member pulleys; and a tension pulley, adjusting a tension of the drive belt. An image write-to-transfer time of the image bearing member which is disposed on a side closer to the tension pulley than the idler pulley is set integer times of a rotation period of the idler pulley.