To cut the corner of a sheet into a rounded shape of high quality at a high speed.

While synchronizing the rotation of a cutter roll 51 with the feed performance of a hooker 42 feeding a work sheet W to the cutter roll side, the work sheet W is fed between the cutter roll 51 and a back up roll 52 and the corner WK of the work sheet W is cut into a rounded shape by a round-shaped cutter blade 511 protrusively formed on the circumferential surface of the cutter roll 51.