PURPOSE: To position a work piece securely and quickly without applying load to its pin by opening and closing chucks by rotation of a cam.

CONSTITUTION: Pins 12-15 acting as the cam followers of cam action pieces 8-11 respectively are projected on the lower faces of chuck cradles 16-19. A work piece is mounted on a work piece receiver 31 and a rotary actuator is operated to rotate a cam. Then the parts of the pins 12-15 where they about against the action pieces 8-11 move gradually to the inside along the action pieces 8-11 and the chuck cradles 16-19 and chucks 26-29 are brought close to the work piece from all directions by action of springs 21 and 25 and chuck the work piece while positioning it. Thereby the work piece can be positioned and chucked securely and quickly with a simple mechanism without applying load to its pin.