To reduce a load in a driving motor by avoiding the setting of an unnecessarily excessive loop gain while improving workability for setting the loop gain.

A loop gain memory part 23 constituting an opening control apparatus C1 memorizes plural loop gains. A loop gain selecting part 24 reads out a loop gain corresponding to a pattern block from the loop gain memory part 23 based on the pattern block obtained from a weaving machine controlling computer Co at each rotation of the weaving machine and outputs it to a control part 25. The control part 25 outputs a control order to a driving circuit 26 based on opening pattern information obtained from the weaving machine control computer Co loop gain information obtained from the loop gain selecting part 24 and the information of weaving machine rotation angle obtained from a rotary encoder 28.