An object of the present invention is to provide a monoclonal antibody which is useful as a diagnostic agent or a therapeutic agent for a disease relating to a polypeptide encoded by Claudin-4 (hereinafter referred to as “CLDN4”) gene or a polypeptide encoded by a Claudin-3 (hereinafter referred to as “CLDN3”) gene, or a method for using the same. Accordingly, the present invention provides a monoclonal antibody or an antibody fragment thereof, which specifically recognizes three-dimensional structure of an extracellular region of CLDN4 and binds to the extracellular region; a monoclonal antibody or an antibody fragment thereof, which specifically recognizes three-dimensional structure of both of extracellular regions of CLDN3 and CLDN4 and binds to the extracellular regions; a hybridoma which produces the antibody; a DNA which encodes the antibody; a vector which comprises the DNA; a transformant obtained by transforming the vector; a process for producing an antibody or an antibody fragment thereof using the hybridoma or the transformant; and a diagnostic agent or a therapeutic agent for a disease relating to a polypeptide encoded by CLDN4 gene and/or CLDN3 gene using the antibody or the antibody fragment.