To provide technique for treatment of fracturing, cleaning, and melting a mass of waste plastic without unraveling it.

The cutter for a mass of waste plastic includes a transportation device 1 for transporting a mass A of waste plastic in a mounted state, a cutter 2 having a cutter blade 21 capable of cutting a mass A of waste plastic provided at the upside of the transportation device 1 and movable toward and away from the transportation device 1, a guide member 22 furnished at the side of the cutter blade 22 in the transportation direction of the transportation device 2 for preventing the cutter blade 21 from being held between the cut surfaces of a mass A of waste plastic, and a driving device 23 for permitting the cutter blade 21 to move toward and away from the transportation device 1 along with the guide member 22.