2 wound on the yarn package 3a is tied to to the yarn wound on yarn package 3b and after the yarn wound on yarn package 3a has been exhausted a supply yarn package change detector 14 detects the trailing end i e a change of the supply yarn package and issues a detection signal to a picking controller 15 referring to fig2 showing the picking controller 15 embodying the present invention the supply yarn package change detector 14 is connected to a delay circuit 16 such as a counter an initial data setting device 17 data updating device 18 a memory device 19 and a control unit 20 which are in turn connected sequentially to the actuator 8 the two input terminals of the memory device 19 are connected to data setting devices 21 and 22 respectively another output terminal of the memory device 19 is connected to a comparator 23 connected to the data updating device 18 on the other hand an arithmetic unit 24 has an input terminal connected to the delay circuit 16 another input terminal connected to the detector 13 and an output terminal connected to the comparator 23 an encoder 25 detects the phase angle of the loom and issues phase data representing the phase angle of the loom to the delay circuit 16 the arithmetic unit 24 and the control unit 20 in the graph shown in fig3 positive values for time t are plotted on the horizontal axis positive values for the length of travel of the free end of the picked weft yarn 2 are plotted on the vertical axis a point s indicates a picking starting position and a point e indicates an arriving position a picking time namely a standard checking pin.