To reduce the abrasion of a lock part and improve the durability by providing a claw driving mechanism which pushes a claw pivoted below the lock part toward a ratchet shaft and locks it, and separates it from the ratchet shaft.

When the ratchet shaft 2 of the fall preventing device A is elevated together with the platen P, the slanting surface of an engagement recessed part 1 presses the slanting surface of the lock part 3a even if the lock part 3a of the claw 3 engages an engagement recessed part 1a, so that the claw 3 leaves the ratchet shaft 1. Although the platen P does not fall when the lock part 3a of the claw 3 and the engagement recessed part of the ratchet shaft 1 engages each other, when the platen P needs to be fallen, compressed air is supplied to a cylinder 5c to rotate the claw 3 on a support shaft 4. Thus, the advance of the abrasion of the claw 3 and ratchet shaft 1 is reduced to improve the durability.