PURPOSE: To prevent a knife from getting on a cord by a method wherein a form of a cutting-off line to be obtained by cutting off a sheet material by causing the knife to run along a scheduled running line is measured and an approximate straight line of the cutting line and a bias angle to be formed by the approximate straight line and a beltlike member are operated.

CONSTITUTION: A knife is moved in the direction meeting at right angles with a running direction along a cord during running and cutting-off, positions of the knife are detected by the first and second detecting devices and a cutting angle is calculated by an operating device. Based on one time or plural times of the latest data, a slewing is driven and controlled by a control device, a running angle of a running member is changed and the running angle of the knife can be made to follow a cutting angle. Therefore, even in the case where a bias angle of a buried cord is changed gradually, the difference between both the angles is made small always and a blade of the knife is made to run along the cord as near as possible by adjusting the running angle of the knife to a changing tendency of the bias angle of the cord.