To provide a round-groove machining method for cutting a round-groove, while moving a rotating tool (a hob, a slab milling cutter, etc.) along a lead, by setting the rotating tool supporting both ends and having many cutting edges in outer peripheral direction and axial direction in parallel to a spiral lead angle to solve problems of round-groove machining having long machining time, rough surface roughness of a machinng surface, a short tool service life and difficult fine adjustments in dimension and groove width.

The rotating tool 1 is set in parallel to a lead angle 4 and the rotating tool 1 is moved along the lead, while synchronizing a cutting feed 5 and rotation 7 of a material to be cut, to cut a round-groove. The fine adjustments of a round-dimension of a round-groove and the groove width are performed by changing the setting angle of the rotating tool 1 for the lead angle 4. By the above method, the spiral round-groove machining by the rotating tool 1 with both ends supported and having many cutting edges in the outer peripheral direction and axial direction is provided.