The present invention relates to a cytokeratin 8/18 complex-specific autoantibody or a fragment comprising an antigen-binding site (paratope) thereof, the use thereof in the diagnosis of breast cancer, a polypeptide having an amino acid sequence of an epitope specifically binding to the autoantibody, a composition for diagnosing breast cancer comprising an agent capable of measuring an expression level of the autoantibody or the fragment comprising an antigen-binding site thereof, a hybridoma cell line producing the autoantibody, and a kit for diagnosing breast cancer comprising the composition of the present invention. Further, the present invention relates to a method for diagnosing breast cancer, comprising the step of detecting the cytokeratin 8/18 complex-specific autoantibody or the fragment comprising the antigen-binding site thereof using the composition of the present invention, and a method for screening a therapeutic agent for breast cancer using the autoantibody.