NOVELTY - The apparatus has circulating-gas line that circulates processed gas to end of biological desulfurization tower. Mixing gas line (5) is provided to mix the biogas with some of processed gas to supply the mixed gas to end of tower. The hydrogen sulfide load amount is calculated from a value measured with gas meter (3) located in biogas inflow line and hydrogen-sulfide densitometer (4) located in mixing-gas line. Circulating gas control mechanism is operated for regulating the amount of circulating gas based on calculated hydrogen-sulfide load amount.    USE - Biological desulfurizing apparatus of biogas.    ADVANTAGE - The hydrogen sulfide contained in high load is efficiently treated and the hydrogen sulfide to be treated is converted into sulfuric acid so as to eliminate clogging within the device and eliminate cleaning steps, and to conduct the treatment at low cost.    DETAILED DESCRIPTION - An oxygen-containing gas inflow line is formed for flowing in oxygen-containing gas into biogas inflow line. A supply control mechanism of oxygen-containing quantity of gas is provided in the oxygen-containing gas inflow line. Signal transmission mechanism of oxygen-containing gas in which the control mechanism of oxygen-containing quantity of gas is operated, based on calculation result of the hydrogen-sulfide load amount. An INDEPENDENT CLAIM is included for a biological desulfurization method involves water-sprinkling a circulation liquid in a biological desulfurization tower from which methane-fermented organic waste is emitted and a hydrogen sulfide biologically is removed. The filled layer consisting of the filler adhered with microorganisms adhered is provided in biological desulfurization tower. The biogas is flowed into the upstream of filled layer in tower. The process gas is discharged to the downstream of filled layer in the tower. The process gas is circulated to the upstream of the filled layer in tower. The biogas with some of processed gas is introduced in the tower in mixture of both. The gas flow rate of the flowing-in biogas in the biogas inflow process and hydrogen-sulfide load amount are computed, based on which the amount of circulating gas of circulating gas is adjusted. The hydrogen-sulfide concentration of the mixed gas is 100-1000 ppm. The hydrogen-sulfide concentration in gas is 150-500 ppm.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the biological desulfurizing-apparatus. (Drawing includes non-English language text)    Gas meter (3)    Hydrogen-sulfide densitometer (4)    Mixing gas line (5)    Arithmetic unit (12)    Flow hydrogen concentration signal input line (16)