a process for the production of a physiologically active peptide containing cysteine residue , comprising the steps of : culturing escherichia coli transformed with a plasmid capable of expressing a fusion protein under control of a promoter of e . coli origin or a promoter of a phage origin , wherein the fusion protein is represented by the following formula : a -- l -- b wherein b represents a target peptide containing cysteine residue ; a represents a partner polypeptide consisting of 90 to 220 amino acid residue but not containing cysteine residue ; and l represents a linker amino acid residue positioned between c - terminal of the partner polypeptide and n - terminal of the target peptide wherein the same amino acid as the linker amino acid is not present in the target peptide , and the linker amino acid is selected so that the peptide bond between the c - terminal of the linker amino acid and the n - terminal of the target peptide is claimed by a protease or the linker amino acid is selectively degraded by a chemical substance ; disrupting the cultured cells and obtaining an insoluble fraction containing the fusion protein ; solubilizing the fusion protein with a solubilizing agent , and treating the solubilizing fusion protein with the protease or the chemical substance to liberate the target peptide , and isolating the target peptide .