Abstract: Provided is a rack transport system that horizontally slides a rack tray holding and storing a plurality of specimen containers, allowing the rack tray to be safely placed on a rack tray set section. For this purpose, a rack transport system (8) using a rack tray (10) having a rack dropout-preventing mechanism (10b) includes a rack tray set section (8A) that has a lock canceling mechanism (8i) of the rack dropout-preventing mechanism and on which the rack tray that arranges and holds a plurality of racks (9) supporting a plurality of specimen containers (9a) is placed, and a projection section (8h) serving as a lock canceling button of the rack dropout-preventing mechanism is formed at a position where the rack tray that holds and stores the plurality of racks supporting the plurality of specimen containers is slid to be able to be set in the rack tray set section.