A ball joint, comprising: a ball stud comprising: a shaft member; and a ball portion formed at one end of the shaft member; and a holder made of a synthetic resin and configured to accommodate the ball portion in a pivotable manner, wherein the holder comprises: an opening portion from which the shaft member protrudes; a first member made of thermoplastic synthetic resin comprising a first ball sliding contact surface having a concave spherical shape approximate to a spherical surface of the ball portion; and a second member made of thermoplastic synthetic resin comprising: a second ball sliding contact surface having a concave spherical shape approximate to the spherical surface of the ball portion; and the opening portion formed in a center of the second ball sliding contact surface, characterised in that the first member and the second member respectively comprise joining surfaces (25, 32) fixed to each other by welding, and respectively comprise fixation reference surfaces (28, 34) that are brought into abutment against each other so that a spacing between the first ball sliding contact surface and the second ball sliding contact surface is maintained constant, wherein the fixation reference surfaces (28, 34) are each formed into an annular shape so as to surround the ball portion of the ball stud, wherein the joining surfaces (25, 32) are formed into an annular shape on an outer side of the fixation reference surfaces (28, 34), respectively, and wherein a step is defined between the fixation reference surfaces (28, 34) and the joining surfaces (25, 32) wherein a gap is defined between the joining surface of the second member and the joining surface of the first member under a state in which the fixation reference surface of the second member is held in abutment against the fixation reference surface of the first member, wherein the joining surface of the first member and the joining surface of the second member are fixed to each other via the molten resin, when the first member and the second member are fixed by welding. a ball stud comprising: a shaft member; and a ball portion formed at one end of the shaft member; and a shaft member; and a ball portion formed at one end of the shaft member; and a holder made of a synthetic resin and configured to accommodate the ball portion in a pivotable manner, wherein the holder comprises: an opening portion from which the shaft member protrudes; a first member made of thermoplastic synthetic resin comprising a first ball sliding contact surface having a concave spherical shape approximate to a spherical surface of the ball portion; and a second member made of thermoplastic synthetic resin comprising: a second ball sliding contact surface having a concave spherical shape approximate to the spherical surface of the ball portion; and the opening portion formed in a center of the second ball sliding contact surface, characterised in that the first member and the second member respectively comprise joining surfaces (25, 32) fixed to each other by welding, and respectively comprise fixation reference surfaces (28, 34) that are brought into abutment against each other so that a spacing between the first ball sliding contact surface and the second ball sliding contact surface is maintained constant, an opening portion from which the shaft member protrudes; a first member made of thermoplastic synthetic resin comprising a first ball sliding contact surface having a concave spherical shape approximate to a spherical surface of the ball portion; and a second member made of thermoplastic synthetic resin comprising: a second ball sliding contact surface having a concave spherical shape approximate to the spherical surface of the ball portion; and the opening portion formed in a center of the second ball sliding contact surface, a second ball sliding contact surface having a concave spherical shape approximate to the spherical surface of the ball portion; and the opening portion formed in a center of the second ball sliding contact surface, characterised in that the first member and the second member respectively comprise joining surfaces (25, 32) fixed to each other by welding, and respectively comprise fixation reference surfaces (28, 34) that are brought into abutment against each other so that a spacing between the first ball sliding contact surface and the second ball sliding contact surface is maintained constant, wherein the fixation reference surfaces (28, 34) are each formed into an annular shape so as to surround the ball portion of the ball stud, wherein the joining surfaces (25, 32) are formed into an annular shape on an outer side of the fixation reference surfaces (28, 34), respectively, and wherein a step is defined between the fixation reference surfaces (28, 34) and the joining surfaces (25, 32) wherein a gap is defined between the joining surface of the second member and the joining surface of the first member under a state in which the fixation reference surface of the second member is held in abutment against the fixation reference surface of the first member, wherein the joining surface of the first member and the joining surface of the second member are fixed to each other via the molten resin, when the first member and the second member are fixed by welding.