PURPOSE: To make highly accurate positioning possible irrespective of a variation in the external dimensions of a work by installing each of contact points, contacting with each other by each one spot at both inner and outer sides of two fulcrum shafts on a straight line connecting these two shafts, in a first arm and a second arm, and giving a function for chucking the work to each contact point at the outside of both these fulcrum shafts.

CONSTITUTION: When a work 6 is chucked, if closing force 7 is added to a lower part than a fulcrum shaft 3 of an arm 1, this arm rotates clockwise with the fulcrum shaft 3 as the center. This motion is transmitted to an arm 2 via contact points 1b, 2b and the work 6. At this time, since it comes to reverse motion as to a fulcrum shaft 4 at the arm 2, finally both contact points 1a, 2a of these arms 1, 2 are joined close to each other and thereby these arms come to standstill in a state of having the work chucked by other contact points 1b, 2b.