Provided herein is a novel screening method for a client protein-protecting protein, and a physiologically active protein-stabilizing protein and a pharmaceutical composition using the protein. Also provided herein is a method for screening for a client protein-protecting protein, including the step of evaluating stability of a client protein in the presence of a protein having an intrinsically disordered structure. Also provided herein is an agent for stabilizing a physiologically active protein comprising, as an active ingredient, a protein having any one of amino acid sequences of SEQ ID NOs: 1 to 6; and a pharmaceutical composition including the protein and a physiologically active protein.