To provide a blade attaching/detaching auxiliary tool capable of fastening or loosening a fixing nut by stipulated torque by rotating the fixing nut without using a torque wrench.

The blade attaching/detaching auxiliary tool includes: a spindle 10; a blade mount 22; the fixing nut 38; and a rotation stopping mechanism for stopping the rotation of the spindle 10. The blade attaching/detaching auxiliary tool further includes: a nut rotation section 70 engaged with an engaged section formed in the fixing nut 38 to rotate the fixing nut 38 in a state the rotation of the spindle 10 is stopped by the rotation stopping mechanism; a nut holding section 80 arranged at an outer periphery of the nut rotation section 70 to hold an outer peripheral section of the fixing nut 38; and a body holding section 90 connected to the nut rotation section 70 via a rotation torque control section 88. The rotation torque control section 88 controls rotary torque applied to the nut rotation section 70 to prescribed torque or below when rotating the body holding section 90 to fasten or loosen the fixing nut 38.