Synthesis gas for FT synthesis is produced using a producing apparatus including an active carbon adsorbing vessel for adsorbing impurities in a natural gas, a hydro-desulfurizer for hydrogenating and desulfurizing sulfur content in the natural gas under a condition of a partial pressure of hydrogen of 100 to 200 kPa, a second hydrogen supplying line for supplying hydrogen to the natural gas between the hydro-desulfurizer and a reactor, the reactor for obtaining synthesis gas by reacting the natural gas, carbon dioxide and steam in the presence of a catalyst for reforming, and a heat recovering boiler for cooling the synthesis gas at a cooling rate of 2000 to 4000.degree. C./second.