To reduce the weight without deteriorating mechanical strength.

A hydraulic flange 20 is constituted of a hydraulic part 21, a flange main body 22, a flange part 23 having the diameter extended on one end of the flange main body 22, and a clamp member 27 for fixing a grinding wheel 26 fitted on the outer peripheral surface of the hydraulic part 21. The flange main body 22 and the flange part 23 are made of a material having the specific gravity lower than that of the hydraulic part 21, for example, aluminum alloy. The hydraulic part 21 made of stainless steel is fitted on the outer peripheral surface 22A of the flange main body 22. An annular plate-shaped hydraulic chamber 32 is formed inside the hydraulic part and near the outer peripheral surface 21A coaxially with the rotational axis O, and an extension part 22a capable of projecting and being displaced toward the outer peripheral side is provided between the hydraulic chamber 32 and the outer peripheral surface 21A of the hydraulic part 21.