To provide a blade mounting support tool capable of mounting an annular blade on a tool mount so that the outer periphery of the blade forms a perfect circle with respect to the center of the tool mount.

The tool mount 12 includes a fitting hole fitted to a rotating spindle, a mounting part having a cylindrical mounting surface extending in the rotating axis direction of the spindle, and an annular receiving part having an annular receiving surface extending from the base end of the mounting part radially outward. The annular blade 2 has a mounting hole fitted to the mounting surface of the mounting part of the tool amount 12. The blade mounting support tool includes a fitting projection 46 to which the fitting hole of the tool mount 12 is fitted, three contact members 60 which are disposed, at equal angular intervals, radially outward of the mounting part of the tool mount 12 fitted to the fitting projection 46 and which have radially outward lengths from the center of the fitting projection 46 set to a same length, and a drive device for radially moving the three contact members 60 in the diameter-reducing and diameter-increasing directions while maintaining, at the same length, the radially outward lengths of the tree contact members from the center of the fitting projection 46.