To provide a cap of a ball joint capable of improving production performance easily with stable quality and the ball joint equipped with this cap.

Protrusions 24, 26 of a protrusion part 27 are protruded on the upper surface 23 and the under face 25 of a cap 14 respectively. Caps 14 are prevented from overlapping with each other at the protrusion part 27 when the caps 14 are plated. The caps can be surely plated, and yield rate is enhanced. Setting direction can be easily checked when the cap is set to a body 12 by eliminating directionality of the front and rear faces of the cap 14. The cap 14 can be set to the body 12 easily. Thereby, the production performance can be improved easily with stable quality.