PURPOSE: To retain fixedly the push-pressing force of a bush plate without relation to the advance position of a spindle head portion during processing and realize the miniaturization of a device by supporting the bush plate by means of cylinders whose fluid pressure is kept fixed.

CONSTITUTION: When a spindle head portion 30 keeps moving toward a work, cylinder strokes contract according to the movement quantity of the spindle head portion 30. At this time, a relief valve 53 functions at all times so as to maintain fixedly pressure in feed passages 51, that is, the internal pressure of cylinders 40, so even if piston rods 41 contact, pressure in feed passages 51 does not rise. As a result, even during the processing of the work, force that push-presses a bush plate 33 and a fixation jig 60 by means of cylinders 40, is kept at fixed pressure whose degree is enough not to deform the bush plate 33 and the fixation jig 60.