The present invention provides a monoclonal antibody which specifically recognizes CD27 containing an O-linked sugar chain to which galactose is not bound and binds to its extracellular region, or a method for using the same. The present invention can provide a monoclonal antibody or an antibody fragment thereof, which specifically recognizes a polypeptide encoded by CD27 gene containing an O-linked sugar chain to which galactose is not bound, and binds to its extracellular region； a hybridoma which produces the antibody； a DNA which encodes the antibody； a vector which comprises the DNA； a transformant obtainable 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 comprising the antibody or the antibody fragment thereof as an active ingredient.
