Sheet post-processing apparatus and image forming system

An image forming system comprising:

• an image forming apparatus that forms an image on a sheet based on image information;
• and a sheet post-processing apparatus, wherein
• the sheet post-processing apparatus comprises;
• a stack tray for stacking the sheet;
• a rear-end-position regulating member, against which a rear end of the sheet stacked on the stack tray is butted to align a position of the sheet in a conveying direction in which the sheet is conveyed toward the stack tray, the rear end of the sheet being a rear end in the conveying direction;
• and
• a roller member that comes in contact with an upper surface of the sheet conveyed to an upper side of the stack tray, before the sheet is stacked on a stacking surface on the stack tray, and conveys the sheet stacked on the stacking surface toward the rear-end-position regulating member, wherein
• a rotation speed before stacking, which is a rotation speed of the roller member when the roller member is in contact with the sheet while the rear end of the sheet in the conveying direction is not landed on the stacking surface, is set to be smaller than a rotation speed after stacking, which is a rotation speed of the roller member when the roller member conveys the sheet to the rear-end-position regulating member while the sheet is stacked on the stacking surface, and
• the rotation speed of before stacking is greater than zero.