PURPOSE: To stabilize opening and closing of a clamping pawl by providing a fixed position limit switch on a clamping final end part and constructing the captioned jig clamping machine such that said machine always initiates its clamping operation from an off state of said switch, in said jig clamping device emplyoing an endless rotating machine.

CONSTITUTION: Both clutches 6, 7 of a jig clamping machine 1 and a workpiece mounting jig 2 are engaged with each other and a hydraulic motor 3 is rotated forwardly. Hereby, an operating shaft 4 is forwardly rotated and a clamping pawl 8 of the jig 2 is alowed to force a workpiece 14 to move in a direction where it is clamped for initiating clamping. Simultaneously, a dog holder 10 also moves to the right, and a clamping dog 10B turns on a limit switch 12 for detecting the clamping end and further said clamping pawl continues it clamping operation until a limit switch 13 for a fixed position is turned on. Then, the hydraulic motor 3 is reversed in a loosening direction to return the switch 13 to its off position. Hereby, said jig clamping machine always initiates its operation from a switch 13-off state without requiring additional stroke redundancy, allowing opening and closing of the clamping pawl to be stabilized.