PURPOSE: To obtain a solid fuel of improved combustion efficiency by mixing a ground PS foam with ground wood, reducing the volume of the obtained mixture and softening the same in a hot air atmosphere to form an indefinite- shape solid fuel containing ground wood and retaining cells.

CONSTITUTION: 100 pts.wt. product of a mean particle diameter of 10-100mm, obtained by grinding waste PS foams, is mixed with 150-500 pts.wt. product of a mean particle diameter of 1-30mm, obtained by grinding waste wood, and the resulting mixture is thrown into a hopper 6. Conveying air is heated with a hot air generator consisting of a blower 1 and a heater 2, and this mixture is fed through a rotary valve 5 at a constant rate. While the mixture passes a particle transfer pipe 3 in a hot air atmosphere at 100-250°C, it is reduced in volume and softened to unite the ground PS foam of a specified gravity of 0.2-0.95 with the ground wood and cooled in a cyclone 4 to obtain an indefinite-shape solid fuel composed of particles of the ground PS foam and a plurality of particles of the ground wood united to the surface of each particle of the ground PS foam and retaining cells therein.