A toner supply assembly includes a toner container mount and a toner container including a container body, a cap having a toner outlet, and a shutter for the toner outlet, to move horizontally. The cap includes a shutter guide rail, a contact portion, and a pressed rail provided on a lateral side. The shutter includes a shutter body to engage the shutter guide rail and a deformable portion deformable vertically and including a stopper to contact the contact portion of the cap for inhibiting the shutter from opening the toner outlet, and a stopper release projection pressed by a bottom projection on a bottom surface of the toner container mount in conjunction with installation of the toner container. A pressing member provided in a side portion of the toner container mount engages the pressed rail of the cap, inhibiting the cap from moving upward.