To reduce a notch generated in the cutting end part in cutting a web by swinging a knife at an inclination of an acute angle to the web.

In a moment when cutting a web 5, an entering angle 18 is formed between a knife 7 and the web 5. This angle 18 is set at most 45°. When adding a vector 15 of a relative speed of the knife 7 to the web 5 and a vector 16 of a relative speed of the knife 7 to a speed component in an optional present position, a synthetic vector having an angle 21 to the vector 16 is obtained. In an ideal state, a cutting end part is adjusted so that a cutting angle 18 becomes the angle 21 between the vectors 15, 16.