To eliminate the need for safekeeping and control by regenerating crushed small paper pieces or the like as useful material.

Cruched small paper pieces, which are not sorted into a combustible article and a non-combustible article, are thrown into an enclosed furnace. The crushed small pieces are heated with a gas burner where no air is supplied into the enclosed furnace, thereby gsasifying the combustible substance in the crushed small pieces. The crushed small paper pieces after heated are continuously melted with a multi-arc in the enclosed furnace. The molten substances are sorted according to each type based on a differential specific weight and taken out. The gas obtained from the gasification of the combustible substance is used as a fuel for a gas burner.