We claim a method of expression a thioredoxin-fused membrane protein comprising expressing said thioredoxin-fused membrane protein in a cell-free protein synthesis system in the presence of a non-ionic detergent. It is preferable that the fused membrane protein which is a highly hydrophobic protein, e.g., a G protein-coupled receptor, can be produced in a state being highly soluble and capable of forming a biologically active three-dimensional structure. Also, this invention provides a recombinant polynucleotide encoding said thioredoxin-fused membrane protein which is expressed in the cell-free protein synthesis system.