The present invention relates to 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.
