PURPOSE: To facilitate the insertion of a core, by pressing high-pressure air into a tube, and by expanding the tube.

CONSTITUTION: The click 3 of a lower mold side diameter-enlargement operating board 4b is closed, and a tube T is set. The click 3 is widened, the diameter of the tube T is enlarged, and the tube T is fitted on the outer diameter of a lower mold 2. The diameter-enlargement operating boad 4b is removed from, and the outer diameter of the tube T is clamped by a clamp 5. A core C is inserted into an upper mold 1, and the click 3 of an upper mold side diameter-enlargement operating board 4a is closed. The lower mold 2 is pulled up, and the inner diameter of the upper end opening edge of the tube T is supported and the diameter is enlarged, and the tube is fitted on the outer diameter of the upper mold 1. From the upper mold 1, high-pressure air is pressed into the tube T. By enlarging the diameter of the tube T, the core C in the upper mold 1 is dropped, and is supported in the lower mold 2 through the interior of the tube T. When the high-pressure air is removed from the tube T and the clamp 5 of the lower mold 2 is removed from and the lower mold 2 is drawn from the tube T, then the tube T shrinks and comes in close contact with the external surface of the core C. The clamp 5 of the upper mold 1 is removed from, and the diameter-enlargement operating board 4a is moved down, and the tube T is removed from the upper mold 1. Then, a tube unit U is obtained.