A method for producing a protein using a cell-free protein synthesis system comprising a detergent so that the protein can be synthesized without aggregation, is provided. The protein is a protein comprising a hydrophobic region in at least a portion thereof, for example, a membrane protein or its fragment (portion). And the detergent is a mild detergent which would not denature the protein, for example, a nonionic or amphoteric ionic detergent.