PURPOSE: To form a joint section satisfying both the length and width at a high speed by concurrently driving a drive source having the short amplitude and cycle and a drive source having the long amplitude and cycle independently provided with a zigzag cutter for forming the sheet joint section.

CONSTITUTION: End sections of two sheets fed from raw-material rolls 3, 4 are cut edge cutters 10, 11 and their joint sections are cut by a zigzag cutter 5. The cutter 5 is shifted to the right and left by concurrently driving motors 14, 17. That is, a locus curve having the amplitude A is formed with the motor 14 having the fast rotation and short reversing cycle and a feed screw 6, and a locus curve is concurrently formed with a motor 17 having the slow rotation and long reversing cycle and a feed screw 15 via a feed nut 16. As a result, a joint width B required fro the bending strength and a joint length required for the tensile strength can be both formed largely.