UNIT OF FLEXIBLE BALL JOINT, METHOD FOR ASSEMBLING THE UNIT, AND METHOD FOR PIPING USING THE UNIT

PROBLEM TO BE SOLVED: To provide a unit of a flexible ball joint which can be easily applied to a tubular passage of a polyethylene tube.SOLUTION: The unit has a first metal made fluid tube 1A having a spherical convex surface 13 wherein the first fluid tube 1A has a first receiving opening part 10A having a curved and concaved inner surface 101 covered by the convex surface 13 in the opposite side of the convex surface 13, a second metal made fluid tube 1B having a spherical concave surface 14 in which the convex surface 13 is fitted and which relatively slidingly rotates around a predetermined rotation center O with respect to the convex surface 13, a first polyethylene pipe 2A having a first inserting opening part 12A, fitted into the first receiving opening part 10A of the first fluid tube 1A with the diameter of the first inserting opening part 12A enlarged, and having a first outer end surface 210 exposed from the first receiving opening part 10A, and a first inner core 4A fitted into an inner peripheral surface of the first inserting opening part 12A for maintaining the state where the diameter of the first inserting opening part 12A the first polyethylene tube 2A is enlarged.