To provide a spacer for a hub bolt capable of mounting wheels having different sizes of reference pitch diameters easily and completely only by inserting the spacer into the hub bolt and fixing it with a wheel nut without working the wheel or changing (processing) the reference pitch diameter.

The reference pitch diameters are formed so as to have almost the same shapes as that of a clearance between the outside diameter of the hub bolt and a hole in the wheel, to ensure that the different sizes of wheels are mounted.