NOVELTY - A low-temperature desulfurizing for desulfurizing a digestion gas is performed using a desulfurizing agent containing iron. Then, a high-temperature desulfurizing of the gas is performed using a desulfurizing agent containing nickel, zinc, or copper. The high-temperature desulfurizing process serves as a pre-heating process for the digestion gas, before reforming of the digestion gas to produce reformed gas.    USE - The desulfurizing method is useful for the desulfurizing of digestion gas produced by methane fermentation, before reforming digestion gas to produce reformed gas for driving combustion engine of digestion-gas utilization system (claimed).    ADVANTAGE - Enables the complete and easy removal of the sulfur content in the digestion gas, by the low and high-temperature desulfurizing processes. Since the digestion gas is pre-heated by the high-temperature desulfurizing process, the reforming process is performed economically and efficiently.    DETAILED DESCRIPTION - The digestion gas is produced by methane fermentation. A combustion engine of a digestion-gas utilization system is driven using the reformed gas. The exhaust heat of the combustion engine is used to increase the temperature of the high-temperature desulfurizing process. The desulfurizing agent containing iron is regenerated by supplying a portion of air supplied to a heat treating furnace (12) to the desulfurizing agent. The utilization system uses the exhaust heat of the furnace as a heat source for the reforming process. The high-temperature desulfurizing agent is regenerated by a portion of steam added to the digestion gas.    INDEPENDENT CLAIMS are also included for:    (1) desulfurizing apparatus of digestion gas; and    (2) digestion-gas utilization system with a methane fermentation tank (10) for performing methane fermentation of organic matter, a desulfurizing towers (16,17) for desulfurizing the digestion gas, and the combustion engine. The main and sub heat exchangers (14,15) pre-heat the digestion gas supplied to the reformer (11) using the exhaust heat of the reformed gas, and heat water by the residual exhaust heat of the reformed gas so as to produce steam, respectively.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of the digestion-gas utilization system. (Drawing includes non-English language text).    Methane fermentation tank (10)    Reformer (11)    Furnace (12)    Main and sub heat exchangers (14,15)    Desulfurizing towers (16,17)