An object of the present invention is to provide a method of efficiently constructing a library abundant in diversity and also usable for screening of a compound that binds to a target substance having protease activity.The present invention provides 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 Xaa0 of a peptide represented by the following formula (I): A-(Xaa0)n-B  (I) [wherein, m numbers of Xaa0s 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 inter selected from 2 to 40, and A and B each independently represent a peptide composed of from 0 to 100 amino acids].