To manufacture slug, having proper strength, water absorption, and a constant size, by a small furnace.

A combustible substance, such as city refuse and industrial waste, is gasified and discharged toward above and ash contents, such as incinerated ash and flying ash, is converted into a melting slag 11 and dropped through a drop opening 2, and a melting furnace 1 is provided to generate the slug 21 by cooling molten slug 11. A rotating rotor 3 is arranged internally below the drop opening 2 and the rotor 3 collides the dropping molten slug 11 with an opposite water-cooled wall 5 through the rotation force of the molten slug 11 for crush, and a control mechanism is provided to produce slug 21, having given grain size, strength, and water absorption.