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 (hereinafter, referred to as “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 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.