To provide a servo mechanism for a clamping device of a rapid and correct stretching machine.

In a stretching machine 10 in which curved clamping devices 24, 25 opposite to each other clamp both ends of a metallic sheet, the clamping devices 24, 25 are grippers which are relatively moved by a hydraulic cylinder and arranged adjacent to each other, the stretching machine 10 includes a closed loop type servo control means, moves each gripper to a specified position, includes a hydraulic cylinder position controller of the hydraulic cylinder of each gripper, and controls the hydraulic cylinder based on the stored data to indicate the specified position for the gripper. The position controller is driven by a motor, a hydraulic cylinder position feedback means is provided on the hydraulic cylinder, and the signal to indicate the position of the cylinder is transmitted to the position controller.