A method of constructing an azoline compound library containing two or more azoline compounds having an azoline backbone introduced into at least one of Cys, Ser, Thr, and 2,3-diamino acid, and analogs thereof, of Xaa 0 of a peptide represented by the following formula: A-(Xaa 0) m -B wherein, m numbers of Xaa 0 respectively represent arbitrary amino acids, at least one of which is an amino acid selected from the group consisting of Cys, Ser, Thr, and 2,3-diamino acid, and analogs thereof, m represents an integer selected from 2 to 40, and A represents a peptide composed of from 2 to 70 amino acids and B represents a peptide composed of from 0 to 30 amino acids, comprising: constructing an mRNA library encoding a precursor peptide comprising, in order of mention from the N-terminus, a sequence 1, -(Xaa 0) m -, and a sequence 2, sequence 1 being composed of from 0 to 10 amino acids, and sequence 2 being composed of from 0 to 10, 11 or 12 amino acids; expressing the precursor peptide in a cell-free translation system by using the mRNA library and thereby constructing a peptide library; and reacting the azoline backbone-introducing enzyme and the peptide library in the presence of a peptide comprising a leader sequence of a substrate of the azoline backbone-introducing enzyme and thereby introducing an azoline backbone into at least one of Cys, Ser, Thr, and 2,3-diamino acid, and analogs thereof, the leader sequence being a leader sequence of a substrate of the azoline backbone-introducing enzyme and composed of from 5 to 50 amino acids, wherein the analog of Cys, Ser, Thr, or 2,3-diamino acid is selected from the group consisting of: wherein, R represents a hydrogen atom, a substituted or unsubstituted alkyl group having from 1 to 10 carbon atoms, or a substituted or unsubstituted aromatic group, wherein at least one of the azoline compounds in the library has an azoline backbone other than oxazoline or thiazoline obtained from Ser, Thr or Cys, and wherein the azoline backbone-introducing enzyme is PatD. constructing an mRNA library encoding a precursor peptide comprising, in order of mention from the N-terminus, a sequence 1, -(Xaa 0) m -, and a sequence 2, sequence 1 being composed of from 0 to 10 amino acids, and sequence 2 being composed of from 0 to 10, 11 or 12 amino acids; expressing the precursor peptide in a cell-free translation system by using the mRNA library and thereby constructing a peptide library; and reacting the azoline backbone-introducing enzyme and the peptide library in the presence of a peptide comprising a leader sequence of a substrate of the azoline backbone-introducing enzyme and thereby introducing an azoline backbone into at least one of Cys, Ser, Thr, and 2,3-diamino acid, and analogs thereof, the leader sequence being a leader sequence of a substrate of the azoline backbone-introducing enzyme and composed of from 5 to 50 amino acids, wherein the analog of Cys, Ser, Thr, or 2,3-diamino acid is selected from the group consisting of: wherein, R represents a hydrogen atom, a substituted or unsubstituted alkyl group having from 1 to 10 carbon atoms, or a substituted or unsubstituted aromatic group, wherein at least one of the azoline compounds in the library has an azoline backbone other than oxazoline or thiazoline obtained from Ser, Thr or Cys, and wherein the azoline backbone-introducing enzyme is PatD.