An object of the present invention is to provide a monoclonal antibody which is useful for treating or diagnosing a disease relating to system ASC amino acid transporter 2 (ASCT2) or a method using the antibody. The present invention provides a monoclonal antibody which specifically recognizes a native three-dimensional structure of an extracellular region of system ASC amino acid transporter 2 (ASCT2) and binds to the extracellular region, or an antibody fragment thereof； a hybridoma which produces the antibody； a DNA which encodes the antibody； a vector which contains the DNA； a transformant obtainable by introducing the vector； a process for producing an antibody or an antibody fragment thereof using the hybridoma or the transformant； and a therapeutic agent using the antibody or the antibody fragment thereof, and a diagnostic agent using the antibody or the antibody fragment thereof.
