PURPOSE: To cope with the grip of the body to be worked in a wide diameter range, by preparing several kinds in advance, fitting a serialized hard claw top jaw to a base jaw, and replacing it.

CONSTITUTION: A master jaw 36, base jaw 37 and top jaw 38 are fitted with their combination to the chuck 30 fitted via an adaptor 35 to the spindle of a machinetool, the master jaw 36 is equipped with a rack engaged with a scroll 32, and composed so as to be driven in the radial direction by the rotation of the scroll 32. Moreover the base jaw 37 is key-combined with the top jaw 38, equipped with the cylinder internal face becoming the reference face for fitting the top jaw and both side guide faces, the top jaw 38 is abutted on the cylinder internal face of the base jaw 37 with the outer end cylinder face, also has a grip face which grips the body 50 to be worked.