PURPOSE: To conveniently form a helical groove, by controlling the moving distance of a spindle attached with a machining tool in the axial direction thereof and the rotating angle of a columnar material held in the horizontal direction, in a predetermined relationship each time when the columnar material is fed by a unit length, orthogonal to the spindle.

CONSTITUTION: A columnar tool material 9 clamped between tail stock bars 8a, 8b is inclined with respect to a spindle 5 so as to make the included angle between the tool material 9 and the rear end face 6a of a grind stone 6 coincident with the twisting angle θ of a helical groove in a lead L. Then, the spindle 5 is moved in the Y-axis direction to be set at a height at which the cut-in depth of the grind stone into the tool material 9 becomes a predetermined groove depth, and further, a bed 2 is moved in the Z-axis direction to be set at a cut-in starting point. Further, the axial moving distance Z of the spindle 5 attached thereto with the grind stone 6 and the rotating angle C of the tool material 9 are controlled by use the formulae l each time when the bed 2 is fed by a division length X of a unit length in the X-axis direction. Thus, it is possible to form a helical groove in a circular material with the use of a horizontal type machining center.