in a grindstone for fabricating a mold which has optical fiber engagement portions of an optical fiber guide block used for aligning optical fibers , two main grinding surfaces are provided to form the optical fiber engagement portions contacted with the sides of the optical fibers and a tip end portion which is contiguous to the two main grinding surfaces and which has a contour contained within a predetermined area . the predetermined area has a triangle shape which is defined by two tangent lines along the two main grinding surfaces and a preselected line drawn between two points determined on the two main grinding surfaces . the two points are decided in accordance with a predetermined formula .