PURPOSE: To prevent thread joining operation at the same time with a plurality of weights by aloways detecting the pressure of an air inhalation source used for operating a thread joining device and transmitting as a thread snap generation electric signal to the thread joining device of each weight to control the operation.

CONSTITUTION: Since an adsorption chamber 5 is in a set negative pressure while the weight is spooling under a normal condition, a switch 10 is opened and a B contact of a relay 16 is always closed. When thread joining operation starts in several places at the same time, necessarily the pressure absorbing force of the adsorption chamber 5 becomes below a set value and the swtich 10 is closed to open the B contact of the relay 16. At this time, if a thread is snapped in the other weights, switches 12, 14 are closed to excite a solenoid 15 and start the spooling operation, but the switch 14 is opened in a short time so that the operation is suspended. At this condition, thread joinings in the other snapped weights are sequentially completed so as to restore the adsorbed pressure to a set value.