An operation method of a synthesis gas reformer of a GTL (gas to liquids) plant having a process for producing synthesis gas by adding at least steam and CO 2 to light hydrocarbon gas to form a mixed fluid and heating the mixed fluid, the operation method comprising: setting an operation condition including control target values of a flow rate of H 2 and CO contained in synthesis gas reformed by the synthesis gas reformer, a H 2 /CO ratio which is defined by a ratio of the number of moles of H 2 contained in the synthesis gas to the number of moles of CO contained in the synthesis gas, a steam/carbon ratio which is defined by a ratio of the number of moles of steam added to the mixed fluid to the number of moles of carbon contained in the light hydrocarbon gas, a CO 2 /carbon ratio which is defined by a ratio of the number of moles of CO 2 added to the mixed fluid to the number of moles of carbon contained in the light hydrocarbon gas, and a temperature of the synthesis gas at an outlet of the synthesis gas reformer; determining control target values of a flow rate of the light hydrocarbon gas, a flow rate of the steam and a flow rate of the CO 2, and an amount of heat needed for the synthesis gas reformer, by the operation condition set, a measured value of the composition of the light hydrocarbon gas, a measured value of the temperature of the mixed fluid at an inlet of the synthesis gas reformer, and a measured value of the pressure of the synthesis gas at the outlet of the synthesis gas reformer; controlling an operation load of the synthesis gas reformer on the basis of the control target values of the flow rate of the light hydrocarbon gas, the flow rate of the steam, and the flow rate of the CO 2; setting a furnace efficiency of the synthesis gas reformer; calculating a combustion load of a burner of the synthesis gas reformer based on values of the furnace efficiency and the amount of heat needed for the synthesis gas reformer; calculating a lower heating value of a fuel gas based on a composition measurement of the fuel gas of the burner; determining a control target value of the pressure of the fuel gas by the combustion load of the burner, the lower heating value of the fuel gas, and a burner performance curve of the synthesis gas reformer; calculating a deviation between the control target value of the pressure of the fuel gas and a measured value of the pressure of the fuel gas; and controlling the temperature of the synthesis gas at the outlet of the synthesis gas reformer by adjusting a pressure control valve provided at an inlet of the burner to compensate for the deviation. setting an operation condition including control target values of a flow rate of H 2 and CO contained in synthesis gas reformed by the synthesis gas reformer, a H 2 /CO ratio which is defined by a ratio of the number of moles of H 2 contained in the synthesis gas to the number of moles of CO contained in the synthesis gas, a steam/carbon ratio which is defined by a ratio of the number of moles of steam added to the mixed fluid to the number of moles of carbon contained in the light hydrocarbon gas, a CO 2 /carbon ratio which is defined by a ratio of the number of moles of CO 2 added to the mixed fluid to the number of moles of carbon contained in the light hydrocarbon gas, and a temperature of the synthesis gas at an outlet of the synthesis gas reformer; determining control target values of a flow rate of the light hydrocarbon gas, a flow rate of the steam and a flow rate of the CO 2, and an amount of heat needed for the synthesis gas reformer, by the operation condition set, a measured value of the composition of the light hydrocarbon gas, a measured value of the temperature of the mixed fluid at an inlet of the synthesis gas reformer, and a measured value of the pressure of the synthesis gas at the outlet of the synthesis gas reformer; controlling an operation load of the synthesis gas reformer on the basis of the control target values of the flow rate of the light hydrocarbon gas, the flow rate of the steam, and the flow rate of the CO 2; setting a furnace efficiency of the synthesis gas reformer; calculating a combustion load of a burner of the synthesis gas reformer based on values of the furnace efficiency and the amount of heat needed for the synthesis gas reformer; calculating a lower heating value of a fuel gas based on a composition measurement of the fuel gas of the burner; determining a control target value of the pressure of the fuel gas by the combustion load of the burner, the lower heating value of the fuel gas, and a burner performance curve of the synthesis gas reformer; calculating a deviation between the control target value of the pressure of the fuel gas and a measured value of the pressure of the fuel gas; and controlling the temperature of the synthesis gas at the outlet of the synthesis gas reformer by adjusting a pressure control valve provided at an inlet of the burner to compensate for the deviation.