An object is to provide a barrel-shaped threaded tool for machining an internal gear which is capable of machining with high accuracy by appropriately setting a helix angle. To achieve the object, in a threaded grinding wheel (11) formed into a barrel shape having a diameter gradually decreasing from an axially intermediate portion thereof toward axially opposite end portions thereof so that the threaded grinding wheel (11) is meshed with a workpiece (W) with a crossed axes angle (Σ) therebetween during the machining of the workpiece (W), the helix angle is set in accordance with the length from the axially intermediate portion.