PURPOSE: To fit a cover of superior recyclable properties close on a container main body by manufacturing a plastic container main body with an expanded diameter section on a mouth section and a plastic dish-shaped cover, heating side sections of the cover, and then inserting the cover to a neck section of the container main body, and blow molding the side sections of the cover.

CONSTITUTION: In a container main body 1, a mouth section 2 consists of an opening end 21 and a more expanded section 22 and a section 23 of contracted diameter formed below the section of the expanded part. A cover 5 is a dish- shaped injection molded body with a side section 52, and the side section 52 is heated by a heater and inserted into the neck section 2 of the container main body 1 in the state of being retained by the cover, and a flange section 51 of the cover 5 is brought into contact with an opening end 21 of the container main body 1. Then, pressurized air is blown into the side section 52 from inside the cover 5 in the state of retaining the container main body 1 by a mouth mold, and brought into contact with a relatively expanded diameter section 22. As the side section 52 of the cover 5 is already heated, the same can be deformed easily.