The present invention is a process to express a target peptide in a large amount and accumulate the target peptide in host cells in the form of inclusion bodies. The process comprises: A) culturing host cells transformed with a plasmid able to express a gene coding for a fusion protein represented in the formula A--L--B, wherein B is a target peptide, A is a protective peptide comprising a 90-210 amino acid fragment E. coli .beta.-galactosidase, and L is a linker peptide positioned between the C-terminus of the protective peptide and the N-terminus of the target peptide and selected so that when the fusion protein is treated by an enzyme or chemical substance, the target peptide is separated, and wherein the protective peptide and linker peptide are selected so that the isoelectric point of the fusion protein in between 4.9 and 6.9; B) obtaining an insoluble fraction comprising inclusion bodies by homogenization of th cultured transformed cells; C) solubilizing the fusion protein in the inclusion bodies by treatment of the insoluble fraction with a solubilizing agent; and, D) cleaving the peptide bond between the C-terminus of the linker peptide and the N-terminus of the target peptide of the solubilized fusion protein to release the target peptide from the other peptides followed by purification of the target peptide.