To provide a flange mechanism having a fixing nut, capable of engaging the fixing nut with a claw part of an attaching/detaching jig, from any one of both front and back surfaces.

The flange mechanism for sandwiching and fixing, from both sides, a cutting blade attached to a tip of a rotatably supported spindle includes: a mounting flange which is composed of a boss part having a male screw on an outer periphery of a tip, and a flange part for supporting the cutting blade, formed projecting in a radial direction from an axial rear end of the boss part, and which is fixed attachably/detachably to the tip of the spindle; and an annular fixing nut 72 having a female screw 74, on an inner periphery, to be screwed on the male screw of the boss part, for sandwiching, with the flange part of the mounting flange, and fixing the cutting blade. The fixing nut 72 has, on an outer peripheral surface, an annular protrusion 76 for fitting onto a claw part 94 of an attaching/detaching jig 80 of the fixing nut 72 to/from the mounting flange, and a shape of the outer peripheral surface is formed in a shape symmetrical with respect to a center of a thickness from a front surface to a back surface of the fixing nut 72.