A method for dressing a tool used for a method for machining an internally toothed gear, the method for machining an internally toothed gear being capable of, even if the grindstone of the tool has a regenerated grinding surface, machining with stable accuracy the internally toothed gear to be machined. A method for dressing a screw-like tool, configured for dressing the screw-like tool by meshing an internally toothed gear-like dressing gear (14), which is rotatable about the dressing gear rotation axis (C2), with the barrel-shaped, screw-like grindstone (11) which has a diameter gradually increasing from both ends thereof in the axial direction thereof toward the intermediate section in the axial direction thereof and which is used for machining an internally toothed gear to be machined. The outer diameter of the screw-like grindstone (11) after dressing is estimated before the dressing, the angle (Sigma2) of intersection between the dressing gear rotation axis (C2) and the screw-like tool rotation axis (B1) of the screw-like grindstone (11) is set on the basis of the estimated outer diameter of the screw-like grindstone (11) after dressing, and dressing is performed with the dressing gear (14) and the screw-like grindstone (11) meshed with each other at the set angle (Sigma 2) of intersection between the axes.