PURPOSE: To carry out the continuous operation under the optimum control by sensing the height of a frost line of a tube-shaped film to be formed, adjusting the discharge volume of external cooled air blown from an annular air ring to the outer face of the tube-shaped film in the case that the frost line is displaced over or under the set value and adjusting the air volume of the tube-shaped film after the completion of automatic adjustment of the frost line.

CONSTITUTION: The height of a frost line of a tube-shaped film F to be formed is sensed by a sensor 36, and in the case that the frost line is displaced over or under the set value, a stop signal for stopping the adjustment of the air volume of the tube- shaped film is output from a stop command section 38 to an air volume adjusting section 33. The discharge of volume of external cooled air is flown from an annular air ring 23 to the outer face of the tube-shape film F by an external cooled air discharge volume adjusting section 39, and the frost line is adjusted automatically to the set height. A stop release signal is output from a stop release section 40 to a stop command section 38 after the completion of adjustment, and the air volume of the tube-shaped film is adjusted by an air volume adjusting section 33 and the inflation gusseted film forming is performed continuously.