To decrease aluminum content in a slag prepared by melting and solidifying incineration ash and sewage sludge and to improve the quality of the slag.

A water granulated slag containing aluminum is crushed by means of a crusher 3 with a pair of rolls 5 the surfaces of which are covered with a rigid rubber layer 5b and then, it is magnetically classified under a magnetic flux density of 2,000-2,800 gauss.