PURPOSE: To obtain the subject compound having serum calcium depressing action on mammals and human on an industrial scale at a low cost by carrying out a peptide condensation reaction while reacting a ring-forming site having protecting group with diphenylphosphoric acid azide, thereby forming a ring.

CONSTITUTION: After synthesizing a straight-chain peptide chain by a peptide condensation reaction, the peptide is dissolved in dimethylamide and the solution is incorporated with diphenylphosphoric acid azide and sodium bicarbonate. The mixture is cooled to 0°C and made to react at 4°C for 2 days to effect the cyclization reaction of the ring-forming site expressed by formula I and obtain a ring expressed by formula II. The solvent is distilled off under reduced pressure, diethyl ether is added to the residue and the formed solid is collected by filtration. The solid is recrystallized with dimethylformamide/diethyl ether to obtain elcatonin expressed by formula III and having serum calcium depressing action on mammals and human in high production efficiency.