The present invention is a method for producing a peptide or a protein in which a side chain contains a modified amino acid residue, which comprises chemically producing a peptide fragment containing an amino acid residue having a modified side chain using an weak acid-cleavable resin, producing a peptide fragment containing no amino acid residue having a modified side chain using a genetic recombination method or/and an enzymatic method, and condensing the resulting two kinds of peptide fragments and, according to the present invention, a peptide or a protein containing modification such as acylation, glycosylation and phosphorylation can be obtained effectively and at high quality.