A method of synthesizing a liquid fuel using a liquid fuel synthesis system, wherein the liquid fuel system comprises: a liquid fuel synthesis portion having a separation membrane permeable to water vapor, which is a by-product of a conversion reaction of a raw material gas containing hydrogen and carbon oxide to the liquid fuel, wherein the liquid fuel synthesis portion being partitioned into a non-permeation side space and a permeation side space by the separation membrane; and a sweep gas supply unit configured to supply the permeation side space with a sweep gas for sweeping the water vapor permeating through the separation membrane, wherein the method comprises: flowing the raw material gas into the non-permeation space through an inflow port (a 1); reacting the raw material gas containing hydrogen and carbon oxide to the liquid fuel in the non-permeation side space; flowing a first outflow gas containing the liquid fuel out of the non-permeation side space through an outflow port; permeating the water vapor, which is a by-product of the conversion reaction of the raw material gas containing hydrogen and carbon oxide to the liquid fuel, through the separation membrane into the permeation side space; flowing the sweep gas from the sweep gas supply unit into the permeation side space through an inflow port; and flowing a second outflow gas containing the water vapor and the sweep gas out of the permeation side space through an outflow port, wherein a temperature of the sweep gas flowing into the permeation side space is higher than at least one of a temperature of the raw material gas flowing into the non-permeation side space and a temperature of the first outflow gas flowing out of the non-permeation side space, and wherein a temperature of the second outflow gas flowing out of the permeation side space is higher than at least one of the temperature of the raw material gas flowing into the non-permeation side space and the temperature of the first outflow gas flowing out of the non-permeation side space. a liquid fuel synthesis portion having a separation membrane permeable to water vapor, which is a by-product of a conversion reaction of a raw material gas containing hydrogen and carbon oxide to the liquid fuel, wherein the liquid fuel synthesis portion being partitioned into a non-permeation side space and a permeation side space by the separation membrane; and a sweep gas supply unit configured to supply the permeation side space with a sweep gas for sweeping the water vapor permeating through the separation membrane, wherein the method comprises: flowing the raw material gas into the non-permeation space through an inflow port (a 1); reacting the raw material gas containing hydrogen and carbon oxide to the liquid fuel in the non-permeation side space; flowing a first outflow gas containing the liquid fuel out of the non-permeation side space through an outflow port; permeating the water vapor, which is a by-product of the conversion reaction of the raw material gas containing hydrogen and carbon oxide to the liquid fuel, through the separation membrane into the permeation side space; flowing the sweep gas from the sweep gas supply unit into the permeation side space through an inflow port; and flowing a second outflow gas containing the water vapor and the sweep gas out of the permeation side space through an outflow port, wherein a temperature of the sweep gas flowing into the permeation side space is higher than at least one of a temperature of the raw material gas flowing into the non-permeation side space and a temperature of the first outflow gas flowing out of the non-permeation side space, and wherein a temperature of the second outflow gas flowing out of the permeation side space is higher than at least one of the temperature of the raw material gas flowing into the non-permeation side space and the temperature of the first outflow gas flowing out of the non-permeation side space. flowing the raw material gas into the non-permeation space through an inflow port (a 1); reacting the raw material gas containing hydrogen and carbon oxide to the liquid fuel in the non-permeation side space; flowing a first outflow gas containing the liquid fuel out of the non-permeation side space through an outflow port; permeating the water vapor, which is a by-product of the conversion reaction of the raw material gas containing hydrogen and carbon oxide to the liquid fuel, through the separation membrane into the permeation side space; flowing the sweep gas from the sweep gas supply unit into the permeation side space through an inflow port; and flowing a second outflow gas containing the water vapor and the sweep gas out of the permeation side space through an outflow port, wherein a temperature of the sweep gas flowing into the permeation side space is higher than at least one of a temperature of the raw material gas flowing into the non-permeation side space and a temperature of the first outflow gas flowing out of the non-permeation side space, and wherein a temperature of the second outflow gas flowing out of the permeation side space is higher than at least one of the temperature of the raw material gas flowing into the non-permeation side space and the temperature of the first outflow gas flowing out of the non-permeation side space.