To reduce the number of operators and to eliminate unevenness due to a personal difference by stretching a tubular film at one side held at a low- speed take-off unit at the other held at a high-speed take-off unit, supplying a pressure gas into the film when arrived at a predetermined heatable state, and starting expanding of the film when the supply of the gas is finished.

After a tubular film 1 is annularly extruded from an extruder, an end of the water-cooled film 1 is inserted from a low-speed take-off unit 2 side, the film 1 is manually held at one side to the unit 2 and at the other side to a high-speed take-off unit 3, and started to be stretched. A detecting step I of detecting whether the film 1 is heatable or not to a stretchable temperature by a temperature detecting means 6, an initializing step II of exhausting a pressure gas from an end 20a of a gas supply tube 20 into the film 1, an intermediate step II of driving the units 2, 3 at a third speed, a stable step IV of driving the units at a second speed, and a gas supply ending step V of finishing filling the pressure gas are sequentially automatically conducted. Thus, the number of operators of starting the stretching can be reduced and unevenness due to a personal difference can be eliminated.