optical sensor for detecting the amount of the weft remaining on the cop and a limit switch for detecting breakage of the weft 4 are arranged at appropriate locations as sensors required for such automatic operation of the device a slack preventing unit 67 is mounted on a fixed frame not shown in the drawings to prevent any slack in the part of the weft 4 extending between the ground fabric and the shuttle 2 when the weft 4 is taken out from the shuttle 2 this slack preventing unit 67 is provided with a fixed ring 68 and a gripper 69 which can grip the weft 4 from inside the fixed ring 68 as illustrated in fig2 and 4 the gripper 69 is coupled with the free end of the piston rod of an air cylinder 70 via an actuator 71 for opening and closing the gripper 69 so that it may be able to be driven vertically in either direction by the air cylinder 70 now the operation of the above described device is now described in the following first of all the free end of the new weft 45 taken out from the new cop 43 stored in the cop storage unit 44 is gripped and pulled out by the weft gripping hand 28 and is hooked upon the hook 49 thereafter the new weft 45 is passed across the locating pins 58 in a.