PURPOSE:To prevent a film from oscillating so as to exclude the occurring of uneven thickness and wrinkling of the film, by providing air holes through a plurality of spaced and secured disc-like bodies, and permitting the inner circumferential surface of a melted resin bulb before it is expanded to be in contact with the disc-like bodies. CONSTITUTION:A thermoplastic resin such as polyethylene and polypropylene is melt-extruded from an annular die 1 to be formed into the bulb 2, the inner circumferential surface of the bulb is caused to be in contact with a stable body comprising a fixed rod 3 and the plurality of the disc-like bodies 6 each having the air hole 8, and the bulb is blown up. With the movement of the bulb, the amount of air that moves to the direction of nip rolls is compensated by the air holes 8, and the degree of the negative pressure in the spaces defined by the bulbs 2 and the discs 6. By adjusting the size of the air holes 8, even a large diameter tubular film can be taken off at high speed, with the flatness of the product better. It is preferable that the configuration of the disc-like body has, for example, a shape 6, 6a or 6b, and the number of the disc-like bodies is 2-6.
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