To smoothly fix a workpiece in a five surface open state.

An annular piston 20 is inserted into the lower part of a guiding hole 17 in a housing 11 so as to be freely moved up and down, and a transmission sleeve 24 is inserted into the cylindrical hole of the piston 20 by opening a first annular gap 21. A ring-like operation tool 36 is inserted into the upper part of the guiding hole 17 by opening a second annular gap 22, and the operation tool 36 is fitted around a plurality of balls 37. At the time of clamping, first a pull bolt 3 attached to a workpiece 1 is inserted into the sleeve 24, and then the sleeve 24 is driven downward by the piston 20. Then, since the operation tool 36 switches the balls 37 to an engaging position X, the driving force of the piston 20 is transmitted through the sleeve 24, the balls 37 and the pull bolt 3 to the workpiece 37, and the reference surface R of the workpiece 1 is pressed to the supporting surface S of a work pallet 7.