A sheet post-processing apparatus according to the present invention has a standby tray for making a sheet at the position above a processing tray. The sheet on the standby tray is dropped and supplied onto the processing tray. This configuration reduces the sheet feeding path from the standby tray to processing tray. The upper end of the center area of a shutter supporting the rear end of the sheet on a sheet discharge tray which is slidable to the sheet discharge section of the standby tray or processing tray is made lower than the upper end of the peripheral area of the shutter to prevent the sheet from being brought into contact with the shutter when the sheet is discharged, thereby avoiding jamming of the sheet and achieving the vertical alignment of the sheet on the processing tray. Movable portions of the shutter are extended when the sheet discharge tray is slid to close the sheet discharge sections of the processing tray and/or standby tray, thereby preventing the sheet on the sheet discharge tray from reversely being fed to the processing tray and/or standby tray.