To provide a method for forming a phase geometric contour at the max. revolving speed of a grinding worm.

A contour forming tool 10 includes a segment 12 for a worm screw part, and its working region 13 is covered with particles of a hard material 16, and its cylindrical portion is put on the shank 26 of a machining tool coaxially. A grinding worm 2 and the tool 10 rotate synchronously during the period the contour is being formed. It is possible to finish a grinding worm flank 1 by desired topology when the tool 10 is moved in the axial direction of the grinding worm 2 with an appropriate correction of the coupling proportion by the CNC control. This enables formation of the contour of a grinding worm 2 corrected phase geometrically at the revolving speed of the grinding worm 2 used in grinding, which has been impossible according to the conventional technique.