Machining coolant/lubricant is used to operate a piston of a clamping-side coupler by feeding same via a coolant feed passage to a clamping pressure chamber through a first check valve and a clamping pressure fluid passage. The same coolant/lubricant is discharged from an unclamping pressure chamber through an unclamping pressure fluid passage and a second check valve. The resulting movement of the piston downwards brings a clamp member into contact with a workpiece to thereby clamp the workpiece on a table. Thus, a dedicated pressure source for clamping can be dispensed with.