A method is provided for producing a membrane protein folded to its native structure or active structure in a lipid disc or a liposome. The method comprises: (a) preparing a reaction solution for cell-free protein synthesis containing a polynucleotide encoding a membrane protein, a steroidal detergent, and a phospholipid, wherein the steroidal detergent is contained at a concentration higher than its critical micelle concentration； (b) decreasing the concentration of said steroidal detergent in the reaction solution； and (c) synthesizing the membrane protein simultaneously with formation of a lipid disk or liposome into which the synthesized membrane protein is integrated.
