A tumor localizing radiation sensitizer, KADT-F10, in which two molecules of nitroimidazole fluorinate, KU2280, are bound to side chain residues of a tumor localizing Mn-porphyrin was synthesized and evaluated. Suppression of tumor growth was significantly higher in a group of mice in which a total 50mg/kg KADT-F10 was administered before radiotherapy at 50 Gly (A: n=7) than in a group of mice in which radiotherapy alone was done (B: n=7). In group B, all mice died within 73 days after radiotherapy, while in group A, the tumor disappeared in five of seven mice, and three of seven mice completely recovered 120 days after radiotherapy. Clear magnetic resonance images of SCCVII tumor were obtained 1.5h after KADTF-10 administration.