In a method of dressing a barrel worm-shaped tool, a disc-shaped disc dresser (13) meshes with a barrel worm-shaped grinding wheel (12) having its diameter gradually increasing from the two end portions in the axial direction thereof towards the middle portion thereof and used to grind an internal gear (11). The barrel worm-shaped grinding wheel (12) rotates around a tool-rotating shaft (B) disposed at a predetermined shaft angle (A1) with a vertical work-rotating shaft (C1) for the internal gear (11). The disc dresser (13) rotates around a dresser-rotating shaft (D), the disc dresser (13) being disposed at an angle equal to a helix angle of the internal gear (11). While the barrel worm-shaped grinding wheel (12) and the disc dresser (13) mesh with each other and are rotating, the barrel worm-shaped grinding wheel (12) and the disc dresser (13) revolve relative to each other within a horizontal plane.