PURPOSE: To improve airtightness, by a method wherein, when a pouring port pipe made of synthetic resin is airtightly connected to a pouring port mouth piece made of a metal, the inner surface of the pouring port pipe is tapered, and the pipe and the mouth piece are interconnected through a rubber ring interposed into the tapered surface.

CONSTITUTION: A pouring port pipe 2 made of synthetic resin is provided at the inner surface of its end with a tapered surface 4 which is gradually increased in an inner size toward the end surface, and is provided with a flange 5 extended from the end surface toward the outside. Meanwhile, a pouring port mouth piece 3 made of a metal is formed such that a flange 12 is secured to the outer periphery of the intermediate part of the cylinder part of the mouth piece, and the cylinder part beyond the flange 12 is interposed into the end of the pouring port pipe 2 with a rubber ring 13 therebetween. The two flanges 5 and 12 are tightened by means of bolts and nuts 14 so that the rubber ring 13 is stretchedly tightened between the flanges 5 and 12 positioned opposite to each other. Further, a coating layer of synthetic resin is applied on the outer surface of the cylinder part of the mouth piece 3 through the medium of an adhesive to improve airtightness between the inner surface of the pouring port pipe 2 and the outer surface of the cylinder part of the mouth piece.