Abstract: An organic matter such as sewage sludge (101) is treated by gasifying the organic matter into a combustible gas and using the combustible gas as a fuel for a gas engine or a gas turbine to recover power (energy). In a method of treating the organic matter, an organic matter as a raw material is dried in a drying process (51), the dried raw material is gasified to produce a generated gas (123), and the sensible heat from the generated gas and/or a combustion gas produced by gasification is recovered by using a heating medium in a recovery process. Then, the heating medium heated in the recovery process is introduced into the drying process as a heating medium gas for drying.