There is provided a technique which can greatly improve throughput at the time when, after a folding process is performed to a bundle of sheets, the bundle of sheets subjected to the folding process is transported to a downstream side, and can prevent a wrinkle from occurring in the bundle of sheets at the time of the folding process. A sheet transport speed of a pair of rollers is changed from a first transport speed to a second transport speed at a specified timing between when a folding blade finishes a pressing operation of the bundle of sheets and starts a return operation to a waiting position and when a rear edge of the bundle of sheets pressed into a nip of a pair of rollers by the folding blade passes through the nip of the pair of rollers.