PURPOSE: To use a substantially cylindrical parison at a clamp line to effectively remove pleats by preforming the cylindrical parison in a shape of a desired rectangular mold, and injecting the air through an opening bottom of the parison.

CONSTITUTION: An initial cylindrical shape of the parison is spread to a substantially rectangular shape 14. The parison 10 is spread, enlarged as it is lowered over a spreading arm 15 by the arm 15 for disposing a shoulder 16 at each corner 17 of the shape 14, and formed substantially in the rectangular sectional shape 14. The air is injected down through an inside of the parison 10 at an opened lowered region 20 of the parison 10 as it approaches to an opening clamp 25 on a mold part 12, and exhausted through the region 10. The parison 10 is spread to the shape 14, and a pinch-off line is sufficiently shortened in the degree connectible with a space usable in the clamp 25.