PURPOSE: To prevent a backlash of an upper rail to a lower rail surely by providing a locking member which has a lock demountable from the locks at the lower rail side following the rotation of a handle.

CONSTITUTION: When a handle 11 is turned to a lock releasing position a locking member 13 is pushed by a pushing means P, and the locking between its lock 17 and the lock 6 of a lower rail 3 is released. When the handle 11 comes to a locking position, the locking member 13 is pushed by the pushing means P, and the lock 17 and the lock 6 of the lower rail 3 are locked, and a stopper 19 of the locking member 13 and a stopper 22 at an upper rail 5 side are also locked. As a result, the upper rail 5 is positioned to the lower rail 3 through the locking member 13, and a backlash is eliminated between the both rails.