PURPOSE: To control the production with good efficiency, by detecting the crosssectional area of an extrudate at all times, comparing the detected crosssectional area with a reference value that is a predetermined value to which the crosssectional area is desirably controlled, and controlling the speed of a take-up by a feed-back method so that the deviation becomes zero.

CONSTITUTION: The crosssectional area of an extrudate is measured and the data is inputted into a speed controller 40. In the speed controller 40, the crosssectional area of the extrudate is compared with a prescribed crosssectional area that is a predetermined value which is aimed, and the take-up speed of the take-up 20 is adjusted so that the deviation becomes zero. The discharge amount Q of the extruder is given by the relationship: Q=K.SV wherein S is the crosssectional area, and V is the take-up speed. Therefore, if the crosssectional area of the extrudate becomes less than the aimed predetermined value, the take-up speed is lowered, while if it becomes higher than that value, the take-up speed of the take-up is increased.