A sheet post-treating apparatus is provided with a discharged sheet stacking tray displaceably provided in a casing portion for stacking thereon sheets discharged from the casing portion, and a holding portion for holding the discharged sheet stacking tray in a first position and a second position overlying the first position, and is designed such that the sheets are stacked on the discharged sheet stacking tray when in the first position, and therefore, when the sheet post-treating apparatus is incorporated into a printer and a cartridge in the printer is to be interchanged with a cartridge cover opened, a user can retract the discharged sheet stacking tray to the second position to thereby interchange the cartridge.