Abstract: The present invention relates to a biomarker set for diagnosing breast cancer comprising two or more protein markers of: apolipoprotein C1, apolipoprotein (a), neural cell adhesion molecule L1-like protein, carbonic anhydrase 1, and fibronectin; a method for detecting the biomarker set in a blood sample through a multiple reaction monitoring; a kit for diagnosing breast cancer comprising antibodies specific to each of the proteins of the biomarker set; and a method for detecting proteins of the marker set in a blood sample through an antigen-antibody binding reaction. The method for detecting the protein marker set in a blood sample by the MRM method or antigen-antibody binding reaction and the diagnostic kit can provide very high accuracy and sensitivity in comparison with the diagnosis method using a single marker and can very conveniently diagnose breast cancer using blood from patients, thereby being effectively used for early diagnosis of breast cancer.