A sheet post-processing apparatus includes a head portion for driving a staple into a sheet bundle and having a base portion and an attachment portion detachable from the base portion; an anvil portion arranged to face the head portion for receiving and bending the staple driven from the head portion; a head portion support member for engaging and supporting the base portion of the head portion; and an anvil portion support member for supporting the anvil portion. A moving device relatively moves the head portion, the anvil portion and the sheet bundle along surfaces of the sheet bundle placed at a stitching position between the head portion and the anvil portion.