To facilitate automatic replacement of a grindstone, in a grinder for grinding a workpiece object by the grindstone covered by a grindstone cover.

A grindstone unit 1 has a fitting interface part 13 fitted to a work spindle 9, a grindstone flange 12 rotating according to the rotation of the spindle 9 integrally therewith, and the grindstone 11 fitted to the peripheral side surface of the grindstone flange 12. A gripping groove GG is provided over the entire circumference at the end of the opposite side to the fitting interface part 13 in the peripheral side surface of the grindstone flange 12. The grindstone unit 1 fitted to the spindle 9 is configured such that the surface of the radially parallel spindle 9 side and a predetermined portion of the grindstone peripheral end are covered with a fixed cover 21 and the opposite side of the spindle 9 is openably covered by an opening and closing cover 22. The automatic replacement of the grindstone unit 1 is performed by gripping the gripping groove GG using a gripper attached to one end of the rotation arm in an automatic replacement mechanism of a tool, while the opening and closing cover 22 is opened.