When a number of print pages, a driving time or a toner adhering amount exceeds a cleaning trigger value during no transfer operation, a circumferential speed of a secondary transfer roller is changed with respect to a circumferential speed of an intermediate transfer belt so as to clean the secondary transfer roller. Particularly, a surface of the secondary transfer roller is totally cleaned when the circumferential speed of the secondary transfer roller is made relatively faster and then slower or vice versa than the circumferential speed of the intermediate transfer belt because the surface of the secondary transfer roller is elastically deformed in both upstream and downstream directions so that the residual toner on the uneven foam surface of the secondary transfer roller is extruded.