PURPOSE: To previously process a yarn and precisely change over the processed yarn at a desired knitting position.

CONSTITUTION: A yarn 12 fed to a weft knitting machine 10 is processed by a yarn processing device 18 to change over properties of yarn. In the yarn processing device 18, it is required to previously operate at an operation position C in order to change over the yarn 12 at a change over position B on a knitted fabric 11. A controller 21 calculates the operation position C from a set value of set circuit 22 and pattern data of memory 23 and when it is detected that yarn feed position to knitting needle attains the operation position C by an encoder 16, the yarn processing device 18 is operated. Length of the yarn from the yarn processing position A to the operating position is nearly equal to sum S of loop length for knitting a part from the operation position C to a changeover position D.