A method for manufacturing a synthesis gas, which makes it possible to procure CO2 inside a synthesis gas-manufacturing plant utilizing the Fisher-Tropsch reaction system, thereby enabling the synthesis gas to be cheaply manufactured by way of the Fisher-Tropsch reaction system without being restricted by the location of CO2 gas source such as an ammonia plant, the synthesis gas comprising a molar ratio of H2/CO suited for synthesizing gasoline, kerosene and gas oil. This method comprises the steps of, feeding a steam-mixed natural gas to a reformer provided with a combustion radiation portion for burning a fuel, the reformer being heated by the combustion radiation portion, recovering carbon dioxide from a combustion exhaust gas generated at the combustion radiation portion, and adding the carbon dioxide to the steam-mixed natural gas at a location on an upstream side of the reformer.