PURPOSE: To prevent overload of a motor by forming the disk blades of the respective rotary cutters into a star shape and forming a top cutting edge so that a specified curve is formed.

CONSTITUTION: When a platelike input material is introduced from the aperture 14 of a hopper 15, the cutting edge of a blade 22a bites the input material according to rotation of the rotary cutters A, B and breakes it. Furthermore when curly metallic scrap, etc., are introduced, these are cut by shearing action of the engaged mutual blades 22a. Further the input material is cut by the mutual action of a blade 20a and a collar 6 opposite thereto. The blades 20a in the disks 20 of the respective rotary cutters are formed into a star shape. The top cutting edge of the blade 20a is lengthened so that it is tilted while forming a projecting curve in the direction of rotation of the disk 20 for the lengthening direction of a generatrix of the disk 20. Thereby the input material can be easily cut.