PURPOSE: To improve the centripetal performance of a pressure bush for pressing a workpiece by providing an annular contact part with magnetic bodies attracted to plural magnetic bodies provided at a supporting plate, and providing an arm part, protruding from the cutout part of a supporting member, with a centripetal plate having an AE sensor.

CONSTITUTION: An annular supporting member 22 formed of a sliding plate 23 of channel cross section and a supporting plate 25 of channel cross section is fitted to the lower face of a pressure foot 20 fitted to the protruding end parts of the action rods 19a of the air cylinders of vertical moving members. Plural magnetic bodies 29 are embedded being spaced by the specified distance in the horizontal supporting wall 25b of the supporting plate 25. A centripetal plate 30 provided with plural second magnetic bodies 31 attracted in correspondence with the polarity of the magnetic bodies 29 is movably supported at the supporting member 22, and a pressure bush 38 having a pressing ring 37 at the lower face is fittingly fixed into the center hole 32 of the centripetal plate 30. The upper face of the arm part 36 of the centripetal plate 30 is provided with an AE sensor 40, and the depth of counter bore is controlled by a servo motor according to the output of the AE sensor.