PURPOSE: To cover simply the inner peripheral surface of a metal joint with a cylindrical body made of synthetic resin.

CONSTITUTION: A pair of synthetic resin made cylindrical bodies 2 having the outer diameter dimension and desired length adapted to the inner peripheral surface of a metal joint 1 are prepared to heat and melt respectively the outer peripheral surface and tip part of the cylindrical body 2. Next, the cylindrical bodies 2 are inserted into both ends of the metal joint 1. While the outer peripheral surfaces of the respective cylindrical bodies 2 are pressed against the inside surface of the cylindrical body 2 to be fused and joined therewith, the tip parts themselves of the cylindrical bodies 2 are butted against each other to be fused and joined with each other. Thus, the inner peripheral surface can be simply covered with the synthetic resin made cylindrical bodies having the uniform wall thickness.