To hold a value indicating a width corresponding to a specified width without necessity of keeping air pressure in a blown film bubble at a const. value or that a blown film web always feeds air which the blown film bubble blows by injection.

To newly perform drawing, blowing is performed into a blown film web 1 between a pair of delivery rollers 6 and a pair of discharging rollers 13 to make a blown film bubble 7. Thereafter, the blown film web 1 is heated in a region where it is transferred to the blown film bubble 7. Water or another fluid 20 with a different b.p. is filled in a film tube 8 and it is recovered above the delivery rollers 6. This fluid is heated at its b.p. or higher until a necessary pressure for the blown film bubble 7 is obtd.