A decladding device of spent nuclear fuel easily and efficiently removes fuel pins from the spent nuclear fuel. Spent nuclear fuel pins P, made of austenitic stainless steel, and in which the spent nuclear fuel is filled, are fed by a feeder 7 one by one or by a unit of several pins and finely sheared into fragments by a shredder 1 having plural rotating blades 4, 5 so that fragments are of a size of 1 to 2 mm or less. The sheared fragments are separated into a fuel pin portion and a nuclear fuel portion by a magnetic separator 20. The magnetic separator 20 comprises a vibration feeder 27 feeding the sheared fragments and is arranged below the shredder 1. Without using a dissolving liquid of nitric acid, decladding is done and the spent nuclear fuel can be efficiently obtained.