A post-processing unit of an image forming apparatus includes a vertical conveying path and a horizontal conveying path. A stapling mechanism and a folding mechanism are arranged in the horizontal conveying path. When saddle processing is carried out, paper sheets sent into the horizontal conveying path are struck against a trailing end stopper and, after the number of sheets to be bound is accumulated, the center of the paper sheets is stapled and folded double. A movable tray projected to the outside of a housing is provided at a terminal end of the horizontal conveying path. The movable tray functions as a tray for discharging paper sheets that pass through a saddle stitcher and also functions as a supporting member that supports a leading end side of the paper sheets at the time of the saddle processing.