To solve the problem wherein a needless stress is caused in a nipple due to interference of the outer diameter of a chamfer of a recess with a spoke hole when the radius of the chamfer of the recess given between an engagement face section (a protrusion) formed in an engagement head section and a shaft-like section is enlarged to avoid stress concentration.

A corner section formed of the engagement face section and the shaft-like section is chamfered by a complex face formed by connecting two chamfer sections of a chamfer section of a recess with a small chamfer radius and a chamfer section of a recess with a large chamfer radius one by one from the engagement face section side.