To detect descending speed of a carriage to prevent it from falling.

A column 1' is equipped with a drum 12 which is energized to rotate in the winding direction of wire 15, a lock plate 17 and a cam plate 21 which are integrated with the drum 12, and a swing plate 23 which is pivoted on the upper part of the lock plate 17. The swing plate 23 has a swing roller 24 on its right side and a tip part 25 which can be engaged with each engaging groove 20 of the lock plate 17 on its left side, and in a normal condition the swing roller 24 contacts with circumferencial edge of the cam plate 21. A locking body 29 which has a locking plate 30 loosely inserted into a supporting pole 26 is pivoted on a carriage 3', a wire 15 and a tension spring 33 are connected together at the upper and lower ends of a connecting plate 32 integrated with the locking plate 30, respectively. In a normal condition, because tensile force of the tension spring 33 is large, the locking plate 30 is pulled downward and is kept horizontally by a stopper 34.