To reduce the friction resistance and accurately move a lock member by providing a pin in the lock member, fitting a collar or a ring around the pin, being rotatable, and engaged in a cam groove.

A pin 25 is displaced along a cam groove 24 and reciprocated for every index operation. A lock member 22 is moved and reciprocated thereby and an index table is locked and unlocked. When the pin 25 is displaced along the cam groove 24, the pin 25 is guided by a collar or a ring 26, and the collar or the ring 26 rotates about the pin 25 and rolls along the cam groove 24. Therefore, its friction resistance is small so as to enhance its speed. The cam groove 24 and the pin 25 are prevented from abraded by the friction resistance and no rattling is generated between the both. The lock member 22 can thus be accurately moved.