It is an object of the present invention to provide a novel antibody capable of specifically recognizing ROBO1 that is expressed in a cell membrane, a hybridoma that produces the above antibody, a method for producing the above antibody, and a tumor diagnostic agent used in PET comprising the above antibody. The present invention provides a monoclonal antibody capable of specifically recognizing ROBO1 existing on the surface of a cell, which is obtained by immunizing an animal to be immunized with a ROBO1-displaying budded baculovirus recovered from the culture supernatant of host cells infected with a recombinant baculovirus comprising the full-length cDNA of ROBO1 as an antigen.