TECHNOLOGY FOCUS - ENVIRONMENT - Preferred Method: The heating medium gas whose humidity has been increased by evaporated water is cooled by treated sewage discharged from a sewage treatment facility to condense water content in the heating medium gas and lower the humidity of the heating medium gas, and the heating medium gas whose humidity has been lowered is reused as the heating medium gas for drying. When the heating medium gas for drying is cooled by the treated sewage, the heating medium gas and the treated sewage are brought into direct contact with each other to cool the heating medium gas and wash away a solid material contained in the heating medium gas with the treated sewage. The generated gas produced by gasifying is introduced into a gas engine or a gas turbine to recover power. Thermal energy is recovered from an exhaust gas discharged from the gas engine or the gas turbine using the heating medium gas. A fuel comprising natural gas, town gas, propane gas, gasoline, kerosene, gas oil, or heavy oil is supplied as an auxiliary fuel or a main fuel to the gas engine or the gas turbine. The amount of fuel supplied to the gas engine or the gas turbine is adjusted so that the sum of the amount of thermal energy recovered from the generated gas and/or the combustion gas that is produced and the amount of thermal energy recovered from an exhaust gas discharged from the gas engine or the gas turbine is equal to or higher than the amount of thermal energy that is required in the drying step.    TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Component: A drying apparatus dries the organic matter. An air preheater performs a heat exchange between the combustible gas produced in the gasification chamber. A drying air circulation passage leads the drying air discharged from the drying apparatus and heated by the air preheater to the drying apparatus again. A condenser condenses away water content in the drying air to lower the rate of the water content in the drying air.