PURPOSE: To prevent dust and cutting oil from entering from outside by covering a chuck body and a master jaw with a dust-proof cover and discharging a work by projecting a knock-out bar retired inside after releasing the work after completion of machining.

CONSTITUTION: An outer peripheral face and a front end face of a chuck body 15 are covered by a dust-proof cover 41 and a through hole 42 to which a work strap metal 34 is projected is formed on the center of the front end face, and window holes 43, 43, 43 are are formed along the radial direction on three positions around the hole 42. The cover is put on the outer peripheral face and the front end face of the chuck body 15 and the space between with the work strap metal 34 is sealed by an O-ring 44 fit to the through hole 42 of the cover 41, and the outer peripheral face of the rear portion of the chuck body 15 and the inner peripheral face of the rear portion of the cover 41 are sealed by an O-ring fit there between and window holes 43 are sealed by an elastic material wiper 46. Air is always supplied into a main shaft and the chuck body 15 to raise internal pressure.