A method is disclosed for dressing a tool used for machining an internally toothed gear. The method includes meshing an internally toothed gear-like dressing gear, which is rotatable about the dressing gear rotation axis, with a barrel-shaped, screw-like grindstone. For machining an internally toothed gear, the outer diameter of the screw-like grindstone after dressing is estimated before dressing, the angle of intersection between a dressing gear rotation axis and the screw-like tool rotation axis is set on the basis of the estimated outer diameter of the screw-like grindstone after dressing, and dressing is performed with the dressing gear and the screw-like grindstone meshed with each other at the set angle of intersection between the axes.