PURPOSE: To facilitate checking whether dressing is completed in all parts of a grinding wheel of internal gear type, enhance the operating time efficiency of a honing machine, and prolong the lifetime of the grindstone by applying a colored layer bearing a color different from the grinding wheel to at least either of the bore surface and cogged part.

CONSTITUTION: The bore surface 2 of a grindstone 1 of internal gear type is dressed by a dress ring and the cogged part 3 is dressed by a dress gear by means of rotational contact. Therein the part of gridstone 1 which has undergone dressing is ground off together with a colored layer, so that it becomes the color of the grinding wheel. On the other hand, the other parts not having undergone dressing owing to distortion, twist. etc., of the grinding wheel 1 is free from grinding-off of the colored layer and remains in the colored condition. Accordingly the operator conducts dressing until he confirms that the colored layer in the part which requires dressing is ground off completely, and therewith excessive dressing can be precluded.