A production method of liquid hydrocarbons from natural gas, comprising: a hydrodesulfurization step of removing sulfur compounds in natural gas by hydrodesulfurization; a synthesis gas production step of producing synthesis gas by way of a reforming reaction of the natural gas after the hydrodesulfurization with steam and/or carbon dioxide; a Fischer-Tropsch oil production step of producing Fischer-Tropsch oil from the synthesis gas by subjecting the synthesis gas produced from the synthesis gas production step to a Fischer-Tropsch reaction; an upgrading reaction step of hydrogenating the Fischer-Tropsch oil produced from the Fischer-Tropsch oil production step; an upgrading gas/liquid separation step of subjecting the hydrogenated product obtained by the upgrading reaction step to gas/liquid separation to obtain liquid hydrocarbons; and a hydrogen separation step of partly branching the synthesis gas produced from the synthesis gas production step to form a branch line at a stage prior to getting to the Fischer-Tropsch oil production step, separating and producing high-purity hydrogen from the synthesis gas entering the branch line and separating the consequently produced residual gas, wherein the high-purity hydrogen separated and produced by the hydrogen separation step is entirely supplied to the upgrading reaction step and employed as hydrogen for the upgrading reaction together with hydrogen containing gas separated in the upgrading gas/liquid separation step so as to make the concentration of hydrogen to be used for the upgrading reaction equal to 96.0 to 97.5 mol%, and wherein the hydrogen containing gas separated in the upgrading gas/liquid separation step is supplied to the upgrading reaction step and also to the hydrodesulfurization step. a hydrodesulfurization step of removing sulfur compounds in natural gas by hydrodesulfurization; a synthesis gas production step of producing synthesis gas by way of a reforming reaction of the natural gas after the hydrodesulfurization with steam and/or carbon dioxide; a Fischer-Tropsch oil production step of producing Fischer-Tropsch oil from the synthesis gas by subjecting the synthesis gas produced from the synthesis gas production step to a Fischer-Tropsch reaction; an upgrading reaction step of hydrogenating the Fischer-Tropsch oil produced from the Fischer-Tropsch oil production step; an upgrading gas/liquid separation step of subjecting the hydrogenated product obtained by the upgrading reaction step to gas/liquid separation to obtain liquid hydrocarbons; and a hydrogen separation step of partly branching the synthesis gas produced from the synthesis gas production step to form a branch line at a stage prior to getting to the Fischer-Tropsch oil production step, separating and producing high-purity hydrogen from the synthesis gas entering the branch line and separating the consequently produced residual gas, wherein the high-purity hydrogen separated and produced by the hydrogen separation step is entirely supplied to the upgrading reaction step and employed as hydrogen for the upgrading reaction together with hydrogen containing gas separated in the upgrading gas/liquid separation step so as to make the concentration of hydrogen to be used for the upgrading reaction equal to 96.0 to 97.5 mol%, and wherein the hydrogen containing gas separated in the upgrading gas/liquid separation step is supplied to the upgrading reaction step and also to the hydrodesulfurization step.