To provide a decorative cap and a wheel nut or a bolt insertion body, and their assembling method.

The insertion body has a hexagonal cross-sectional part and a cross-sectional part having a cylindrical outside surface with coating and/or plating applied. The cap has a hexagonal cross-sectional part and a cylindrical cross-sectional part. The hexagonal cross-sectional part of the cap easily slides on the hexagonal cross-sectional part of the insertion body. An inner diameter of the cylindrical cross-sectional part is formed smaller than an outer diameter of the cylindrical insertion body, and thus, when the insertion body is pressed in the cap, interference fit is established between the inner diameter of the cylindrical cross-sectional part and the outer diameter of the cylindrical insertion body.