To prevent the occurrence of vibration caused by the eccentricity of a hub blade attached by fitting this in a spindle shaft by providing an air film forming means for forming an air film with an annular inner peripheral part in an annular supporting part for supporting the annular inner peripheral part of the hub blade.

When a hub blade 4 is attached to a spindle shaft 5, the hole 4c of a wheel part 4a is positioned being fitted in the outer peripheral surface of a shaft part 12 and the annular inner peripheral surface of the hub blade 4 is naturally supported by the annular supporting part of the spindle shaft 5. By high pressure air jetted out from the jetting-out port 18 opened in the shaft part 12, a uniform air film is formed in a gap between the annular supporting part and the annular inner peripheral surface. By engaging a mount nut 14 with a screw part 13 so as to fix the hub blade 4 while eccentricity is corrected by this air film, the hub blade 4 is fixed to the tip of the air spindle 5.