To enable an apparatus for post-processing a sheet to easily and suitably carry out operations of aligning a plurality of sheets each including an image formed thereon to subject it to post-processing, and making a sheet discharging member discharge the post-processed sheets in a small space.

A tray 11 for use in post-processing is separated into a first tray member 111 disposed on the front side and into a second tray member 112 disposed on the rear side. The first tray member is disposed movable in the width direction of sheets. The first tray member is moved in the direction of approaching the second tray member to align the mounted sheets in the width direction thereof at width-direction aligning members 111a, 112a, and to allow a discharge-direction aligning means 13 disposed on the first tray member to align the sheets in the discharging direction. The aligned sheets are stapled by a stapling device 14 disposed on the first tray member, and subsequently the first tray member is moved in the direction of separating from the second tray member. The stapled sheets are discharged from above the tray for use in post-processing to a sheet discharging member 6 of an image forming apparatus 1.