PURPOSE: To enhance sealing properties of the title mouth, and to reduce variations in the size of the outer diameter of the same due to the variation of temperatures by a method wherein a drum plate is heated and set in a first bending mold provided with a semi-circular processing groove to be pressed, and then a second bonding mold provided with a processing groove in circular arc is set to continue the pressing, thereby forming a protruding part in substantially complete circular arc around the mouth.

CONSTITUTION: One edge side formed in the longitudinal direction of a drum plate 13 made by cutting a pipe is inserted between inner and outer molds 21 and 22 for heating and heated from both sides of the inner and outer molds by a heater 23 so that a bending portion 13a of said edge side can be easily deformed. The bending portion 13a thus heated is then curved by a press in semi-circular arc outwardly along the shape of a processing groove 24c of a first bending mold 24. Then, another second bending mold 25 is set on the upper surface of the base part 24b of the first bending mold 24, and the drum plate 13 is further depressed by the press so that the top end of the bending portion 13a is curved in circular arc inwardly along a processing groove 25a. Thus, by curving the bending portion 13a in circular are to such a degree that said bending portion almost abuts against the outer circumferential surface of the drum plate 13, a protruding part 20 in substantially complete circular arc, which surrounds the ouber circumference of an opening 15 of the drum plate 13, can be formed.