In a so-called GTL process of producing synthesis gas from natural gas, producing Fischer-Tropsch oil by way of Fischer-Tropsch synthesis of the obtained synthesis gas and producing liquid hydrocarbons containing fuel oil by upgrading, the synthesis gas produced from the synthesis gas production step is partly branched off prior to getting to the Fischer-Tropsch oil production step and the synthesis gas entering the branch line is subjected to a water gas shift reaction to raise the hydrogen concentration thereof. Subsequently, high-purity hydrogen is isolated from the synthesis gas and the residual gas left after the isolation is circulated to the synthesis gas production step and used as raw material for synthesis gas production. As a result, a significant improvement can be achieved in terms of raw material consumption per product of the entire process.