PURPOSE: To surely aud firmly fix a workpiece having an unparallel surface, protrusion, or the like by rotatably providing a rotary ball having a contact plane in one or both opposed surfaces.
CONSTITUTION: A workpiece W is positioned with a machining surface 20 facing upward between rotary balls 15, 15, to be pressed to a condition with a slight margin left by operating a pressing mechanism 14 with a moving jaw 13 facing a fixed jaw 12. While fine adjusting the machining surface 20 of the workpiece W to a condition suited for machine work, the workpiece is pressed to a firm condition by the pressing mechanism 14, to complete fixing. In the fixing, even when a clamped surface 21 of the workpiece W is not necessarily a parallel surface, by rotating the rotary ball 15 in accordance with tilting the clamped surface 21, the clamped surface 21 can be surely further firmly fixed in a condition that a contact plane 15b of the rotary ball 15 is brought into surface contact.