A liquid fuel synthesis system includes a liquid fuel synthesis portion and a sweep gas supply unit. The liquid fuel synthesis portion is partitioned into a non-permeation side space and a permeation side space by the separation membrane. 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 a first outflow gas flowing out of the non-permeation side space. A temperature of a 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.