PURPOSE: To lock definitely the position of a locking bar member not only in a locked state but also in an unlocked state.

CONSTITUTION: A locking space 4 is formed in one piece with a horse shoe type casing 1 where a locking member 6 is provided in this locking space 4. An arc-shaped locking bar member 2 housed in the casing 1 is energized with a spring 3 while notched parts 8 and 9 are formed at locations in the locking bar member 2. When the locking bar member 2 is placed in a locked position, the locking member 6 is adapted to penetrate one notched part 8 so that it may be mechanically locked. When the locking bar member 2 is placed in an unlocked state, the locking member 6 is adapted to penetrate the other notched par 9 so that it may be mechanically locked.