Peptide synthetic method to suppress deletions caused by diketopiperazine formation

The present invention addresses the problem of a desired elongation reaction not progressing as a result of a 6-membered cyclic amidine skeleton structure and a diketopiperazine formed when removing an N-terminated protective group during solid-phase peptide synthesis. The inventors have discovered that it is possible to solve said problem by modifying the production of a peptide via the solid-phase method so as to treat a peptide protected by an N-terminated amino group in a protective group having an Fmoc skeleton by using a base which has a pKa of 23 or higher in an acetonitrile of a conjugate acid in a specific solvent, and next, to perform peptide chain elongation.