It is intended to provide a process for efficiently producing a ligand binding protein characterized in that this protein or its domain is synthesized in a cell-free protein synthesis system containing a ligand that binds to the protein or its domain. In a preferred embodiment, the above-described ligand is a substance selected from the group consisting of DNAs, RNAs, nucleosides, nucleotides, proteins, peptides, aminoacids, lipids, carbohydrates, derivatives thereof, alkaloids, terpenes, low-molecular weight compounds, coenzymes, low-molecular weight metal complexes and metal ions, while the ligand binding protein or the ligand binding domain thereof is synthesized as a soluble protein.