Between June 1994 and February 2009, a total of 4, 504 patients with malignant tumors were registered in phase I/II dose-escalation studies and clinical phase II trials using carbon ion beams generated at the Heavy Ion Medical Accelerator in Chiba (HIMAC). Carbon ion radiotherapy (CIRT) has a unique advantage due to the biological properties and excellent dose distribution for malignant tumors compared with other radiotherapy. The cell mortality rate from a carbon ion beam is higher than that from a photon beam when the same physical dose is irradiated. This cell killing effect ratio is expressed by the relative biological effectiveness (RBE), and the RBE of a carbon ion beam has been reported to be 2-3, meaning that the cell killing effect of a carbon beam is two to three times stronger than that of a photon beam. In terms of the physical characteristics, CIRT exhibits a spread-out Bragg peak (SOBP) and makes for a better dose distribution of the target volume by specified beam modulations. In the initial dose-escalation studies, grade 3 or more late rectal complications had developed in some patients. However, the adverse effects were resolved because of the use of appropriate dose levels and modification of the radiation technique. CIRT can carry out hypofractionated radiotherapy with a large fraction dose and reduce the overall treatment times compared with conventional radiotherapy. It can also achieve better local tumor control even for radio resistant tumors such as malignant melanoma, hepatocellular carcinoma and bone and soft tissue sarcomas with minimal morbidity to the normal surrounding tissues. It was concluded that CIRT is an effective local treatment for patients with malignant tumors for whom surgical resection is not a viable option, and it presents a promising alternative to surgery.