NOVELTY - The apparatus comprises a supply tube for supplying a biogas from a biogas repository, a first concentration measuring equipment installed at the supply tube to measure an initial concentration of the biogas, an anaerobic digester, a unit for removing carbon dioxide in the biogas, a water removal unit for removing a moisture content in the biogas, a siloxane removing apparatus that removes siloxane in the biogas, an ammonia removal apparatus for removing the ammonia content in the biogas, and a second concentration measuring equipment installed at a discharge pipe of the ammonia removal unit.    USE - The apparatus is useful for preprocessing a biogas for a fuel cell (claimed), a gas engine, a gas turbine, a steam turbine and a boiler.    ADVANTAGE - The apparatus effectively removes the impurity material included in the biogas in a simple manner.    DETAILED DESCRIPTION - The apparatus comprises a supply tube for supplying a biogas from a biogas repository, a first concentration measuring equipment installed at the supply tube to measure an initial concentration of the biogas, an anaerobic digester, a unit for removing carbon dioxide included in the biogas, a water removal unit for removing a moisture content in the biogas, a siloxane removing apparatus that selectively removes siloxane in the biogas, an ammonia removal apparatus for removing the ammonia content included in the biogas, a second concentration measuring equipment installed at a discharge pipe of the ammonia removal unit to measure the concentration of the biogas from the ammonia removal unit, a desulfurizer for removing hydrogen sulfide in the biogas, a manometer that measures a compression pressure of the biogas, and a third concentration measuring equipment installed at a discharge pipe of the desulfurizer to measure the concentration of the biogas from the desulfurizer. The desulfurizer comprises a compressor that compresses the biogas. The siloxane removing apparatus is divided into a first siloxane removing unit and a second siloxane removing unit. The desulfurizer, the ammonia removal apparatus and the siloxane removing apparatus remove hydrogen sulfide, ammonia and siloxane through an activated charcoal, respectively. The concentration measurement equipments are visibly arranged in order to read the measurement results. A control valve controls the flow of the biogas in the supply tube and the discharge pipe. A controller controls the operation of the apparatus based on the current concentration and pressure of the biogas.