NOVELTY - Carbon dioxide and hydrogen sulfide present in the sewage gas produced during methane fermentation are separated using an alkali absorption solution. The alkali absorption solution is regenerated from the exhaust gas of the electricity generation unit.    USE - Used for generating electricity using sewage gas produced during fermentation of organic substances such as biomass, organic waste, foodstuff waste, stock raising waste water, etc.    ADVANTAGE - Methane concentration stabilization, desulfurization of digested gas and carbon dioxide separation are performed simultaneously. Simplification and miniaturization of the electricity generation system is realized. Methane gas is recovered so energy efficiency is improved.    DETAILED DESCRIPTION - The regeneration unit and the absorption unit have respective upper and lower reservoirs which are connected through a liquid communication path. The digested gas is introduced into the reservoirs of the absorption unit so that the gas contacts with the alkali absorption solution flowing from the upper reservoir of the absorption unit. The carbon dioxide and the sulfide gas are absorbed by the absorption solution. The absorption solution is circulated from the lower reservoir of the absorption unit to the lower reservoir of the regeneration unit, subsequently. The flow rate of the exhaust gas introduced in the regeneration unit is adjusted so that the methane concentration of the purified gas is fixed. The gas emitted out from the regeneration unit is led to the raw-food desulfurization unit and raw-food oxidative degradation of the hydrogen sulfide in the gas is performed. The electricity regeneration unit has a solid polymer fuel cell or phosphoric acid mold fuel cell. Electricity regeneration is performed using gas turbine or gas engine, and the exhaust gas is ejected from the turbine or the gas engine.    An INDEPENDENT CLAIM is included for electricity generation system which has a gas-liquid contact material package placed between the upper and lower reservoirs of the absorption unit. A vapor-liquid separator is positioned in the upper reservoir and a regeneration off-gas outlet is placed at the upper portion of the vapor-liquid separator. A detector detects the methane gas concentration and carbon dioxide concentration on downstream of the purified gas outlet. A cooling unit and a heating unit are provided at the upper and lower reservoirs of the regeneration unit.    DESCRIPTION OF DRAWING(S) - The figure shows the block diagram of the electricity generation system. (Drawing includes non-English language text).