PURPOSE: To shorten the length of dafective elongation percentage by calculating elongation percentage by making the unit measuring length of elongation percentage short when the absolute value of deviation between the target value of elongation percentage and a measured value of elongation percentage is large and making it long when the absolute value of deviation is small. CONSTITUTION: A belt-like matter 1 is rolled with work rolls 2a, 2b as being subjected to tension with an bridle roll 4 on the inlet side and bridle roll 5 on the outlet side. The pulse from a pulse generator 6 on the inlet side which is provided on the shaft of the bridle roll 4c on the inlet side is counted with a pulse counter on the inlet side and inputted to a unit 10 for calculating elongation percentage. An outside pulse is similarly inputted, the absolute value of deviation is calculated from a calculated elongation percentage and the target value of elongation percentage with an arithmetic unit 11 for the absolute value of deviation of elongation percentage, the unit measuring length L of elongation percentage is calculated with the arithmetic unit 12 and outputted to the arithmetic unit 10 for elongation percentage. When the absolute value of deviation of elongation percentage is larger, the length of defective elongation percentage is shortened by hastened responsiveness in the control system of elongation percentage by shortening the unit measuring length L of elongation percentage.
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