A hinge ball for a variable capacity wobble plate compressor includes a central through hole 27 for permitting a shaft 5 to loosely extend therethrough, and an outer peripheral wall 9b in a convex form for being slidably fit with an inner peripheral wall 43a in a concave form of a boss 43 of a drive hub 41 rotatively fitted on the shaft 5. The shaft 5 is higher in hardness than the hardness of the boss 43 of the drive hub 41, and the outer peripheral surface 9b in sliding contact with the inner peripheral surface 43a of the boss 43 of said drive hub 41 is lower in hardness than the hardness of the shaft 5 and the boss 43 of the drive hub 41 or equal in hardness thereto. The hardness of the inner peripheral wall 9a defining the central through hole 27 is made higher than the hardness of the boss 43 of the drive hub 41.