To improve the perpendicularity of a grinding tool or a cutting tool and a hydraulic flange.

This hydraulic flange is composed of a flange body 22, an enlarged flange part 23 on one end of the flange body 22, and a clamp member 27 for fixing a grinding wheel 26 fitted on an outer peripheral surface of the flange body 22. An approximately annular hydraulic chamber 32 is formed coaxially with a rotary axis O at a position opposite to an inner peripheral surface 24B of a base metal 24 near an outer peripheral surface 22A. An expanded part 22a projectable and displaceable to an outer peripheral side is formed between the hydraulic chamber 32 and the outer peripheral surface 22A of the flange body 22. An approximately annular clamp part hydraulic chamber 42 is formed coaxially with the rotary axis O at a position opposite to the other end surface 24D of the base metal 24 near an end surface 27D inside of the clamp member 27. A clamp part expanded part 27a projectable and displaceable in the direction in parallel to the rotary axis O is formed between the clamp part hydraulic chamber 42 and the end surface 27D of the clamp part 27.