An object of the present invention is to provide a method for producing a peptide library capable of incorporating an arbitrary number of arbitrary proteinogenic and/or non-proteinogenic amino acids in an arbitrary site. The invention provides a method for producing a peptide library including 1×106 or more kinds of peptides containing amino acids encoded by N1N2N3, including a step of preparing an mRNA library including mRNAs which encode peptides of the peptide library and each contain at least one N1N2N3; and a step of translating each mRNA of the mRNA library in a cell-free translation system added with tRNA containing an anticodon to any one of N1N2N3 codons and charged with an amino acid corresponding to the codon (wherein, N1, N2, and N3 are each independently selected from adenine (A), guanine (G), cytosine (C), and uracil (U) and an arbitrary amino acid is reassigned to each N1N2N3).