A sheet post-processing apparatus discharges and conveys a sheet from a process tray to a storage tray, and decelerates a bundle hook and a paper discharge roller when the bundle hook reaches a discharge start position of a bundle of sheets. At this time, a tangential speed of the paper discharge roller is faster than a movement speed of the bundle hook. Subsequently, the sheet post-processing apparatus stops the paper discharge roller in accordance with a rotation position of the bundle hook, and then decelerates the movement speed of the bundle hook. After the sheets are stacked on the storage tray, the bundle hook accelerates to return to a home position. Additionally, the sheet post-processing apparatus includes a spool 17 having a sheet pressing guide member 16 with a bent portion 16A. When the sheets are conveyed to a process tray 13, the spool 17 is rotated forward to longitudinally align the sheets and then is backwardly rotated to press the sheets using elastic force of the sheet pressing guide member 16, thereby preventing a gap from occurring between the sheets or preventing the sheets from being misaligned.