To simplify the structure of a locking mechanism and enhance the reliability by arranging a sliding latch with a slidable locking claw energized outward by a built-in spring and disposing a stopper at a position corresponding to the locking claw in front of the rack body.

A sliding latch 12 is constituted of a case 18, a spring 19 built in the case 18, and a locking claw 13 which is energized outward by the spring 19 and is slidable. The locking claw 13 is provided with a sloping surface 13a and a perpendicular surface 13b of e.g. 45, and a stopper 20 and an operating knob 21 are formed integrally with the locking claw 13. Though the locking claw 13 is energized outward (upward) by the spring 19, the projecting position is controlled by the stopper 20. For instance, when the knob 21 is pushed to the arrow direction with a finger inserted in the opening 22, the locking claw 13 is moved downward against the repulsion of the spring 19.