To provide an admixture material and a covering material, both decreasing abrasion in a sliding part.

Alumina is impregnated with a liquid hygroscopic material (e.g., sodium sulfonate) to give an admixture material 18. The admixture material 18 is mixed to give a porous resin (e.g., thermoplastic elastomer), which is molded to give a cover 14. This cover 14 is used, together with grease 16, for covering a joint section. Thus, the infiltration of water into the joint section is effectively inhibited, resulting in the decrease of the abrasion of a sliding part.