To simply obtain configuration of a required formed tool.

By indicating respective planes 4 orthogonal to a disc tool shaft 3a in the coordinate system of a die 1, respective crossing lines of a helical gear curve 2 crossing respective planes 4 can also be indicated by the coordinate system of the die 1. Points 6 where the distances between the disc tool shaft 3a and the respective planes 4 are the shortest are found. Respective points 6 being the point on respective crossing lines, it is possible from the very nature to make vector indication by the die 1 coordinate system. The disc tool shaft 3a also being indicated by the die 1 coordinate system, positional relations (du, dv) between respective points 6 and the adjoining points can be indicated by the die 1 coordinate system by corresponding them to the disc tool shaft 3a. Curves connecting respective points 6 together are grinding lines 7 necessary for grinding the helical gear curve 2. Thus a preferable disc tool configuration can be obtained by transferring the relation of the shaft 3a to the grinding lines 7 as it is as a grinding wheel contour configuration curve of the disc tool.