An image forming apparatus includes a transferring belt, photosensitive drums, primary transferring parts, a transfer voltage applying part, a resistance value measuring part, a cleaning device, a pre-brush, a pre-brush current detecting part and a transfer voltage correcting part. The pre-brush applies a cleaning charge voltage to the remained toner at an upstream side from the cleaning device in the rotation direction of the transferring belt. The transfer voltage correcting part, by a subject resistance value of the primary transferring part measured by the resistance value measuring part, corrects a transfer voltage of the primary transferring part. As a measurement condition, if a change quantity of the pre-brush current from the last transfer voltage correction is equal to or more than a predetermined change threshold, the resistance value measuring part executes measurement of the subject resistance value and the transfer voltage correcting part executes correction of the transfer voltage.