PURPOSE: To change the length of web cutting easily and in short time by stabilizing distance between the first and the second cuttings, and changing a fixed coupling type clutch set on a driving system of a cutting blade to change the phase of a cutting blade.

CONSTITUTION: Distance l1 is made stable between the first and the second rotary blades (6, 6a), (7, 7a) set in the moving direction of a web W, and in order to cut a web W in length b, one cluth out of fixed coupling type clutches 15, 16 set on a driving system of a rotary blade is operated and the other clutch is off, cutting thus a web W in length l1 with the first and the second rotary blades. In order to change the cutting length of a web W from l1 to l2, the clutch 15 is set off and the other clutch is set on, the phase of a rotary blade 7 is changed for the amount of l2-l1, and a feed roll 25 of a web W is driven by pulleys 28, 31 with a clutch 34 off, a clutch 35 on, and the running speed of a web W is conformed with a periphery speed of a rotary blade, the web W being cut in length l2.