A color laser printer includes a plurality of photosensitive drums and a paper-conveying belt disposed in confrontation with the photosensitive drums. The paper-conveying belt conveys a sheet of paper as the photosensitive drums sequentially transfer toner images thereon. This printer also includes a tension-adjusting mechanism capable of adjusting the tension state produced in the paper-conveying belt, and a controller for controlling the tension-adjusting mechanism. Through the control of the controller, the tension-adjusting mechanism is capable of adjusting the tension state of the paper-conveying belt between a first tension state used for transferring toner images from the photosensitive drum, and a second tension state producing a tension greater than that produced by the first tension state for performing a cleaning operation.