A threading method for threading a work 18 includes a supporting portion moving-up step for moving a supporting portion 17 upward by a predetermined distance by using an elevational mechanism 15 without rotating a tap 20 inserted into the work 18. Thus, the bent state of the robot arm 11 can be eliminated by moving the supporting portion 17 upward. In this case, since the supporting portion 17 is moved upward artificially, the supporting portion 17 can be moved up in a short time. That is, the bent state of the robot arm 11 can be eliminated in a short time and so the processing efficiency of the threading step can be improved.