Abstract: A mainframe case includes a sliding cover connected thereto through sliding engagement of first-stage sliding rails slidably mounted in two fixed channels on the mainframe case with second-stage sliding rails fixedly mounted on the cover corresponding to the fixed channels. An intermediate sliding rail is mounted between the first-stage and second-stage sliding rails to slide relative to the first-stage and the second-stage sliding rails. To open the cover for maintenance, repair, and replacement of hardware in the mainframe case, simply pull it relative to the mainframe case in a predetermined direction, and the first-stage sliding rails, the intermediate sliding rails, and the second-stage sliding rails are sequentially brought to slide in the same direction to locate the cover at an open position without completely separating from the mainframe case. And the cover may be conveniently pushed in reverse direction to slide back to a close position again.