Abstract: The present invention relates to a switching binder binding to an antigen-binding portion of an antibody, a preparation method thereof, and a pharmaceutical composition, an assay kit, and an antigen and antibody assay method, each utilizing the same. A switching binder according to an embodiment of the present invention includes an amino acid sequence selected from framework regions of a heavy chain or a light chain, which constitutes an antigen-binding portion of an antibody, wherein the amino acid sequence includes, in addition to the heavy chain or the light chain, a first antibody-binding portion or a second antibody-binding portion that binds to a framework region of a heavy chain or a light chain, which constitutes the antigen-binding portion, the amino acid sequence being able to bind specifically to the antigen-binding portion of the antibody and then be separated from the framework region when a target antigen is provided.