A printer includes a main body capable of printing and outputting paper, a bracket attached to the main body, two sliding blocks, and a tray configured for receiving the paper. The bracket includes two sidewalls. A retaining member is secured to each sidewall. The sliding blocks are slidably attached to the sidewalls of the bracket. The tray is received in the bracket and has tray posts corresponding to the retaining members. The sliding blocks bring the tray to slide in the bracket. The retaining members have a first position, where the tray posts urge the retaining members to slide before passing across the retaining members, and a second position, where the tray posts are blocked by the retaining member when the tray is released from the sliding blocks.