To improve the durability of a timing pulley and its lock mechanism while the conventional security is kept.

A lock pin holder part 7 is integrally formed on a lock arm 3, and a lock pin pierced through the inside of the above part in such a manner as to freely slide is energized toward a pulley by a spring 9. At the time of locking, the tip of the lock pin 8 is fitted to a recessed part of a timing pulley to lock the rotation of the timing pulley. In the thus locked state, even if the lock arm is pushed from a gap by a metal scale or the like, a lock release preventing piece 6 provided on the lock arm is obstructed by a lock release plate 5 so that the lock arm 3 is not moved. As the tip of the lock pin 8 is brought into contact with the pulley recessed and projecting surface with moderate force by the energizing force of the spring 9, even if the tip of the lock pin 8 runs on the projected part of the timing pulley at the time of locking, the lock mechanism is not broken.