A sheet post-processing apparatus (1) includes a processing tray (5), a sheet discharge port (7), a conveyance rollers pair (4), lower and upper discharge rollers (81), (82) and a discharge tray (9). When the sheet having a folded portion is stacked on the processing tray (5) with the folded portion downward, a controller rotates the lower and upper discharge rollers (81), (82) to advance the folded portion to the sheet discharge port (7), stops the lower discharge roller (81) before the folded portion passes the sheet discharge port (7), rotates the conveyance rollers pair (4) and the upper discharge roller (82) so as to convey the rear half portion to the downstream side until the sheet passes the conveyance rollers pair (4) and to form a deflection on the sheet, rotates the lower and upper conveyance rollers (81), (82) and then to stack the sheet on the processing tray (5).