PURPOSE: To enable the shearing of a plate of very large thickness during its stoppage and make driving power sources compact, by using the independent driving power sources to operate a moving mechanism for shearing cutters and a shearing mechanism for a workpiece to be cut off.

CONSTITUTION: A motor for 21 for synchronizing the movement of a workpice 26 to be cut off is driven so that the speed of the movement of the workpiece is equal to that of the uniform-velocity part of a curve which is the locus of the upper end of a vertical connecting rod 23. The moving speed of the upper cutter 7 on an upper side frame 27 coupled to the upper end of the vertical connecting rod 23 is made coincident with that of the workpiece 26 at the uniform-velocity part of the curve of the rod 23. A shearing motor 10 is synchronously driven to start shearing the workpiece 26 at the beginning of the uniform-velocity part. The workpiece 26 is sheared by the cooperation of the upper cutter 7 on the upper cutter side frame 27 with a lower cutter 9 on a lower cutter frame 8 while the workpiece is moving.