PURPOSE: To provide the diameter expansion method which can improve remarkably a service life of formed rubber, at the time of executing the diameter expansion of a metal tube and a synthetic resin lining metallic tube by a bulge forming method.

CONSTITUTION: In a state that a seal ring 4 made of a synthetic resin, which has almost the same diameter as the inside diameter of a tube 2 to be subjected to diameter expansion, and is formed by a material being softer than hardness of the inner wall surface of the tube is allowed to run along the end face of formed rubber 3, and also, a metallic washer 5 whose diameter is smaller than that of the seal ring is allowed to run along the outside of the seal ring 4, pressure is applied to the forming rubber 3 through the washer 5 and the seal ring 4.