A sheet post processing apparatus of compact shape with good alignment can be obtained by providing an accumulation tray for aligning and accumulating sheets successively delivered, a post processing unit for performing a post process such as binding to a sheet stack accumulated on the accumulation tray, and a grip unit for gripping the post processed sheet stack. The post processing unit enables the accumulation tray to reciprocate in a direction transverse to a discharging direction of a sheet discharge tray. The grip unit has a grip part for gripping the sheet stack, a guide part for reciprocally guiding the grip part back and forth in the sheet discharging direction. The grip part is separable from the guide part during a movement of the post processing unit.