To provide a mounting structure wherein an arm is fitted to a screw shaft and fastened thereto with a nut while eliminating the dispersion of the mounting angle of the arm to prevent the backlash of the arm.

An I-cut hole 32 of the arm 3 is fitted to an I-cut portion 23 of the screw shaft 22 and the nut is threaded to the screw shaft 22 to fasten the arm 3, whereby the arm 3 is mounted on the screw shaft 22. In the I-cut portion 23, a tapered portion 25 is formed so that the I-cut hole 32 is closely adhered to the tapered portion 25 with the operation of the nut to fasten the arm 3. Thus, the dispersion of the mounting angle of the arm 3 is eliminated to prevent the backlash of the arm 3.