A sheet post-processing apparatus which is capable of preventing a stapling position and a folding position from being out of alignment when a stapling process is carried out. A sheet bundle, which is stacked in a housing guide, is stapled by a stapler using a staple at the stapling position. The sheet bundle is folded at a position of the staple after the stapling. After the stapling, the sheet bundle is moved from the stapling position to the folding position. During the movement, the staple is detected using a staple sensor disposed at a predetermined location between the stapling position and the folding position. Based on a moving distance of the sheet bundle before the staple is detected after the stapling process, a position of the sheet bundle with respect to the stapler for stapling on a subsequent sheet bundle is corrected.