TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The biomass material pelletized has a particle size not exceeding 100 mm. The densified pellets of the size-adjusted biomass material have a water content of 80-85% by mass on the wet basis. Preferred Process: The extrusion of the biomass material is carried out through a die having a diameter of 7-100 mm under a pressure of at least 0.5 MPa. The pelletization of the biomass material is conducted by extrusion of the biomass material through a die. The biomass material for pelletization is blended beforehand with calcium hydroxide or calcium oxide (0.001-5 mass.%). The methane fermentation is followed by recovery of the fermentation residue which is dried for use as a solid fuel. The solid fuel is heated with superheated steam followed by heating at greater than or equal to 800 degrees C to produce a thermal decomposition gas. Preferred Device: The acid fermentation tank is provided with a feeding pit for the biomass material connected to a conveyer, an extruder having a die shaped at the tip of it, and an upwardly slanted bottom surface connected to the extruder through the die. The methane fermentation tank is connected to the acid fermentation tank through a movable valve which is opened or closed by an angle-controlling actuator. The methane fermentation tank is provided on the upper surface of it with a press-compacting head and a bottom surface shaped to be aslant downwardly in direction of a slurry reservoir; and a recirculation mechanism for returning a slurry collected in the slurry reservoir to the methane fermentation tank. The metal-removing mechanism is further provided for a preliminary treatment of the biomass material. The methane fermentation tank is provided at a discharge port of the fermentation residue with a drying oven. EXAMPLE - A solid matter (water content: 40% wet basis) obtained by pulverizing and dehydrating garbage was mixed with slaked lime (0.01 mass.%) and the mixture was shaped into 3 kinds of pellets each having a diameter of 7 mm and a length of 20 mm and having apparent densities of 650 kg/m3, 800 kg/m3 and 900 kg/m3. The pellets were compressed under pressure of 1 MPa and supplied to a two-phase type solid-state methane fermentation apparatus system of 200-liter capacity at a feed rate of 0.5 kg/day, while a calculated amount of water was sprayed over 2 days to an acid fermentation tank leading to the open air in such a manner that the amount of water at the fifth day might become 70% wet basis. Similarly a comparative process was also carried out without compressing the pellets under pressure. In this state, the pellets from both processes were allowed to stay for 5 days without addition of water. The amount of additional water at methane fermentation tank was calculated and the amount of water was adjusted to become 85% wet basis. The temperature of the methane fermentation tank was continuously maintained at 25 plus or minus 5 degrees C. By the way, the surface temperature of the pellets in the acid fermentation tank at the fifth day was measured and found that the cumulative volume of biogas evolved in each sample was increased by 20-30% in the process as compared to the comparative.