PURPOSE: To enable a complete combustion of solid fuel made from a waste plastic material and a combustible material to be carried out by a method wherein a combustion chamber for a primary combustion and a secondary combustion is arranged on a grille in a combustion cylinder and a combustion chamber for a third combustion is arranged on the former combustion chamber.

CONSTITUTION: A grille 2 is installed at lower part within a combustion cylinder 1 and a space within the combustion cylinder 1 on the rostrum is divided into a combustion chamber 5 and an aeration chambr 6. An aeration port 9 is opened at a side wall 101 of an aeration chamber, a surrounding atmosphere is passed into the aeration chambe 6 and solid fuel to be supplied from a fuel storing chamber 7 to the combustion chamber 5 is smoothly supplied. A combustion chamber 8 to be arranged over the combustion chamber 5 is a combustion chamber for use in performing a third combustion and this is contructed by a sub-combustion chamber 801 and a main combustion chamber 802. With this arrangement, the solid fuel made from waste plastic material and combustible material shows a complete combustion and at the same time almost of all the smokes are not discharged. The solid fuel in a fuel storing chamber 7 is not overheated due to the presence of the aeretion chamber 6, the waste plastic material does not show any viscosity and then the fuel is supplied smoothly to the combustion chamber 5.