A tapered cutting edge portion and a continuous raised portion from the tapered cutting edge portion are ground in an edge portion of a straight grinding raw material so that an eccentricity is not generated. In a method for grinding a treatment device, one end portion of a tapered grinding raw material is inserted between a grinding surface of a grindstone and a pressing surface of a press block, and finishing surfaces opposite each other while having a predetermined dimension, shape, and angle are ground so that a direction in which abrasive grains of the grindstone run corresponds to a direction of a shaft center. At this point, before the finishing surfaces are ground in the predetermined dimension, shape, and angle which are previously set corresponding to an objective reamer, pre-grinding is performed while a necessary finishing margin with respect to at least one of the surfaces, and then finishing grinding is performed to the other surface.