The present invention relates to an epitope of a thioredoxin-1 (Trx1) antigen and a use thereof, and more particularly, to the epitope, and an antibody or an antigen-binding fragment binding thereto. The epitope region of the human Trx1 antigen confirmed in the present invention may be effectively used in the development of an improved antibody to enhance the binding affinity of an anti-Trx1 antibody. In addition, the improved antibody of the present invention is effective in improvement of performance of a breast cancer diagnosis kit due to excellent binding affinity for Trx1 and very high sensitivity and specificity, compared to a conventional anti-Trx1 antibody. Further, the accuracy and reliability of breast cancer diagnosis may significantly increase because exceptionally high sensitivity and specificity are exhibited by detecting the monoclonal antibody of the present invention, which specifically binds to Trx1, rather than detecting CA15-3, another conventional breast cancer diagnostic biomarker.