A sorter, which sorts sheets transferred from an image forming apparatus to form a plurality of sheet stacks and staples each sheet stacks, is disclosed herewith. The sorter is provided with a plurality of bins arrayed in an upright direction for stacking the sorted sheets, and is provided also with a stapler to staple each sheet stack. The bin is adapted to swing horizontally between a first angular position to stack the sorted sheets and a second angular position to undergo the stapling of the sheet stack by the stapler. The stapler is adapted to move along the upright direction through a plurality of stapling positions, and staples each sheet set at each stapling position.