Recently, particle beam radiotherapy with protons or carbon ions has been used in cancer treatment. Energy deposition with particle beams increases as depth increases. Furthermore, carbon ion beams have stronger biological effects than X-rays or proton beams, because carbon beams generate denser ionization along the pathway of the particles. In Japan, clinical study with carbon ions for cancer therapy was initiated in 1994 at the National Institute of Radiological Science(NIRS). Four treatment facilities are now in operation, including Gunma University Heavy Ion Medical Center. The experience with carbon ion radiotherapy at NIRS has demonstrated advantages for the following types of cancer. In terms of histological type, adenocarcinomas, sarcomas, and melanomas that are relatively radioresistant to conventional X-ray radiotherapy may be sensitive to carbon ion radiotherapy. Primary sites that may be sensitive include the head and neck region, lung, liver, prostate, bone and soft tissue, and pelvis(for recurrence of rectal cancer). Combined with surgery, cytotoxic drugs, molecular targeted drugs, and immunotherapy, carbon ion radiotherapy promises to be an important modality in the future.