PURPOSE: To enlarge the application range of a jig and to improve the work efficiency including arrangement, by clamping the body to be worked held on an indexing part with a clamp arm by the signal of a sensor on the way of a cutting tool approaching to the body to be worked.

CONSTITUTION: A sensor 9 transmits a signal with the approach of a cutting tool 8 on the way, when a rotating main shaft 21 is descended by a handle 21a. With this signal a relay 17 operates a solenoid valve 18 to feed a compressed air being fed to the clamp motion side of a cylinder 5, rotating a clamp arm 7 in an arrow mark 30 direction and clamping the body 10 to be worked existed in a work position by the V-shaped groove 7a of the tip. On descent of the cutting tool 8 further, the body 10 to be worked can be processed. The signal of the sensor 9 disappears, when the cutting tool 8 is kept away from the body 10 to be worked by ascending the cutting tool 8 by the handle 21a and the relay 17 becomes in a no excitation state. The compressed air under feeding to the solenoid valve 18 is therefore changed over to a clamp release side and the clamp of the body 10 to be worked by the arm 7 is released.