To provide a container which enables accurately welding of a top upper wall surface of the body of the container an internal wall surface and a partition of a mouth part by improving drawing ability under a molten condition to draw the wall surface by suction when the mouth part is welded on the body of the container.

This container is made up of the body 1 thereof and a mouth part 2 comprising a material mainly composed of polyvinyl chloride. In the material making up the body 1 of the container, 2-5 wt.pts of an acrylic copolymer with an average molecular weight of 1,000,000 or more is added to the polyvinyl chloride per 100 wt.pts thereof to improve drawing ability under a molten condition. In the method of welding the mouth part of the container, there are arranged a process in which a bottom part of the mouth part 2 crowned with a mouth holding jig 5 (having a suction pipe 10 protrusively arranged therein with a suction pad 7 mounted on the outer circumference of a suction port 11) at an upper part thereof is pressed and welded onto a top upper wall surface of a molten tube 1B composing the body 1 of the container and a process in which the suction pad 7 is brought closer to a suction hole 8 formed in a partition 6 of the mouth part to make a suction through the suction port 11 so that the top upper wall surface of the body 1 is drawn in the direction of the suction hole 8 to be weld on an internal wall surface and the partition 6 of the mouth part 2.