A method for manufacturing a ball joint comprising: a ball-seat molding step for forming a ball seat assembly by using, as a core, a ball section of a ball stud, the ball section of which is integrally provided in advance at one end portion of a stud section, and insert-molding a ball seat made of resin to cover at least a portion of the ball section present on an opposite side to the stud section, and a housing molding step for insert-molding a housing made of resin by using, as a core, the ball section partially covered by the ball seat of the ball seat assembly, characterized in that the housing includes, in an opening section (11f3), a taper section (11f1) having a recessed shape widening toward another end portion of the stud section, and the housing molding step includes: an inner-piece disposing step for disposing a divided inner piece (13, 13A, 13B), which forms the taper section (11f1), such that the divided inner piece (13, 13A, 13B) comes into contact with a part (10b1) exposed from the ball seat of the ball section present on one end portion side of the stud section; an inner-piece contacting step for closely contacting the inner spherical surface (13a1, 13b1) of the divided inner piece (13, 13A, 13B) to the part (10b1) exposed from the ball seat of the ball section by pushing an inclined surface (10a3) opposite to the ball section of the ball stud with a pushing means passing through the divided inner piece (13, 13A, 13B) such that the ball stud is moved to a side of the stud section; a step for insert-molding the housing; and an inner-piece detaching step for, while moving the divided inner piece (13, 13A, 13B) in a direction of the other end portion of the stud section, rotating the divided inner piece (13, 13A, 13B) to separate a portion of the divided inner piece (13, 13A, 13B) located on the other end portion side of the stud section from the stud section and detaching the divided inner piece (13, 13A, 13B). a ball-seat molding step for forming a ball seat assembly by using, as a core, a ball section of a ball stud, the ball section of which is integrally provided in advance at one end portion of a stud section, and insert-molding a ball seat made of resin to cover at least a portion of the ball section present on an opposite side to the stud section, and a housing molding step for insert-molding a housing made of resin by using, as a core, the ball section partially covered by the ball seat of the ball seat assembly, characterized in that the housing includes, in an opening section (11f3), a taper section (11f1) having a recessed shape widening toward another end portion of the stud section, and the housing molding step includes: an inner-piece disposing step for disposing a divided inner piece (13, 13A, 13B), which forms the taper section (11f1), such that the divided inner piece (13, 13A, 13B) comes into contact with a part (10b1) exposed from the ball seat of the ball section present on one end portion side of the stud section; an inner-piece contacting step for closely contacting the inner spherical surface (13a1, 13b1) of the divided inner piece (13, 13A, 13B) to the part (10b1) exposed from the ball seat of the ball section by pushing an inclined surface (10a3) opposite to the ball section of the ball stud with a pushing means passing through the divided inner piece (13, 13A, 13B) such that the ball stud is moved to a side of the stud section; a step for insert-molding the housing; and an inner-piece detaching step for, while moving the divided inner piece (13, 13A, 13B) in a direction of the other end portion of the stud section, rotating the divided inner piece (13, 13A, 13B) to separate a portion of the divided inner piece (13, 13A, 13B) located on the other end portion side of the stud section from the stud section and detaching the divided inner piece (13, 13A, 13B). an inner-piece disposing step for disposing a divided inner piece (13, 13A, 13B), which forms the taper section (11f1), such that the divided inner piece (13, 13A, 13B) comes into contact with a part (10b1) exposed from the ball seat of the ball section present on one end portion side of the stud section; an inner-piece contacting step for closely contacting the inner spherical surface (13a1, 13b1) of the divided inner piece (13, 13A, 13B) to the part (10b1) exposed from the ball seat of the ball section by pushing an inclined surface (10a3) opposite to the ball section of the ball stud with a pushing means passing through the divided inner piece (13, 13A, 13B) such that the ball stud is moved to a side of the stud section; a step for insert-molding the housing; and an inner-piece detaching step for, while moving the divided inner piece (13, 13A, 13B) in a direction of the other end portion of the stud section, rotating the divided inner piece (13, 13A, 13B) to separate a portion of the divided inner piece (13, 13A, 13B) located on the other end portion side of the stud section from the stud section and detaching the divided inner piece (13, 13A, 13B).