PURPOSE: To easily conduct the releasing operation of a lock mechanism for stopping a case with a single hand while holding the case.

CONSTITUTION: An engaging groove 2a on the lower surface of a rail 2 and an engaging part 22 on the upper surface of a case 10 are formed in such a manner that they are laid in the engaged state or non-engaged state depending on the horizontal rotating position of the case 10. The case 10 has a lock mechanism to prevent the rotation of the case 10 in the engaged state. The lock mechanism is formed of a slide stopper 15 made possible to vertically slide on a part of the side wall of the case 10, and has a projection 18 entering into the engaging groove on its upper end. When the case is rotated in the position forming the engaged state, thus, the slide stopper 16 is raised and the projection 18 enters into the engaging groove to lock the case.