PURPOSE: To keep a weight value of a flat tube per unit length at a set value, by a method wherein a weight value per unit length of one roll is set up beforehand, resin weight to be supplied to an extruding machine per unit hour is measured in order, the measured value is divided by a set up value whenever the value is measured and a peripheral speed of a receiving roller for the flat tube is controlled in order based on the quotient.

CONSTITUTION: A weight value per unit length of one roll is set up beforehand, resin weight to be supplied to an extruding machine 12 per hour is measured in order, the measured value is divided by a set value whenever the value is measured, a peripheral speed of a receiving roller 13 for a flat tube is controlled in order based on this quotient and the weight value per unit length of the flat tube to be received by the receiving roller 13 is kept at the set up value.