Disclosed is a library primarily comprising a plurality of antigen-binding molecules having different sequences and having at least one amino acid residual contained in the antigen-binding domain, said amino acid residual varying the binding activity of the antigen-binding molecules with respect to the antigens, according to the ion concentration conditions. Also disclosed are: a composition containing a plurality of polynucleotide molecules that code antigen-binding molecules; a composition containing a plurality of vectors that contain the aforementioned polynucleotide molecules; a method for selecting the aforementioned antigen-binding molecule; a method for isolating the aforementioned polynucleotide molecules; a method for preparing the aforementioned antigen-binding molecules; and a pharmaceutical composition containing the aforementioned antigen-binding molecules.