To provide a ball joint which ensures enhancement of the load resistance without resulting in a large size construction.

A plurality of load receiving ribs 27 are protruded radially on the outside surface of the bottom part of a bearing seat 4 confronting the mounting surface part 11 of the bottom surface 8 of a housing 2. Between the load receiving ribs 27, auxiliary ribs 28 whose protrusion height is smaller than the lead receiving ribs 27 are in parallel furnished protrusively without being continued to the load receiving ribs 27 through a recess. The stress applied to the bearing seat 4 from the sphere head part 20 is absorbed by the pressing energy of a restoring force owing to the elasticity and the dimensional tolerance of each component member is absorbed, and thereby the friction compensating function is given. When a large load is impressed, the load receiving ribs 27 are deformed, and the area between these ribs 27 undergoes a deflective deformation, and the auxiliary ribs 28 contact with the mounting surface part 11. Also the load is received by these auxiliary ribs 28, to suppress the deflective deformation of the area between the load receiving ribs 27, which permits prevention of fatigue destruction even though a large load is impressed repetitively. Thus the load resistance and the durability can be enhanced.