NOVELTY - An air injection apparatus (13) injects air into biogas. The biological and dry-type desulfurization apparatus (11,12) de-sulfurize the biogas using microorganisms and iron oxide respectively. A flow sensor (31) detects flow volume of de-sulfurized biogas. A concentration sensor (32) detects concentration of methane contained in de-sulfurized biogas. A temperature sensor (33) detects temperature in dry-type desulfurizing-apparatus. A control apparatus (41) sets air injection rate of air injection apparatus based on detected values of sensors.    USE - Purification system of biogas for electric power generation system.    ADVANTAGE - Since the control apparatus sets air injection rate of air injection apparatus based on detected values of flow sensor, temperature sensor and concentration sensor, the ratio of the air with respect to biogas can be set appropriately. Therefore, decline in the desulfurization efficiency of the biogas by biological and dry-type desulfurization apparatus can be prevented, and biogas can be purified stably.    DETAILED DESCRIPTION - The air injection apparatus injects air into biogas. The biological and dry-type desulfurization apparatus de-sulfurize the biogas using microorganisms and iron oxide respectively. A flow sensor detects flow volume of de-sulfurized biogas. A concentration sensor detects concentration of methane contained in de-sulfurized biogas. A temperature sensor detects temperature in dry-type desulfurizing-apparatus. A control apparatus sets a priority to sensors in order of temperature sensor, flow sensor and concentration sensor, and uses detected value of each sensor from high to low priorities. The control apparatus sets air injection rate of air injection apparatus based on detected values of sensors. The detected value of the temperature sensor is compared with a predetermined value when the control apparatus sets the air injection rate of the air injection apparatus. An air injection rate is decreased from the present setting value if the detected value of the temperature sensor is larger than predetermined value else an air injection rate is maintained to the present setting value, and the amount of biogas generations computed based on the detected value of the flow sensor is compared with preset value. The air injection rate is increased from the present setting value if amount of biogas generations is larger than preset value else the amount of biogas generations is compared with another predetermined value. The air injection rate is maintained to the present setting value, if amount of biogas is more than predetermined value else the air injection rate is decreased to the present setting value, and the detected value of the concentration sensor is compared with another preset value. The air injection rate is maintained to the present setting value if the detected value of the concentration sensor is more than preset value else air injection rate is decreased from the present setting value.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of the biogas purification system. (Drawing includes non-English language text)    Biological desulfurization apparatus (11)    Dry-type desulfurization apparatus (12)    Air injection apparatus (13)    Flow sensor (31)    Concentration sensor (32)    Temperature sensor (33)    Control apparatus (41)