chambers 47c and 47d the valve 47e is normally in the position shown in fig2 so that the chambers 47c and 47d are connected together when the valve 47e is energized the communication between the chambers 47c and 47d is interrupted the cylinder 47a is formed with an elongated slot 47f extending longitudinally along the wall of the cylinder 47a but sealingly closed by the piston 47b the piston 47b is connected with the gripper 46 through a connecting rod 47g passing through the slot 47f a spring 47h is provided in the cylinder 47a for biasing the piston 47b rightward in the plane of fig2 the gripper 46 has a hydraulic clamp 48 which includes a pair of clamping arms 48a and 48b and a hydraulic actuator 48c for actuating the clamping arms 48a and 48b the hanger 2 has vertically extending posts 2a and the clamp 48 is arranged so as to grip one of the posts 2a between the clamping arms 48a and 48b the gripper 46 is normally in the extreme right position on the carriage 45 under the influence of the spring 47h the carriage 45 is also in the extreme right position on the base 41 in the extreme right position of the clamp 48 there is a post detector 49a which is adapted to detect the post 2a on the hanger 2 at the left and of the carriage 45 there is a workpiece detector 49b the control unit 5 receives detection signals from the detectors 49a and 49b further the control unit 5 receives a conveyor speed signal from a speed detector 1a when the post 2a of the hanger 2 is detected by the detector 49a and the detection.