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.