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Heavy ion cancer therapy by using energetic carbon ions has been successfully under way at HIMAC, Japan. The method is advantageous over the traditional radiation therapy by using photons and electrons in two points: dose concentration owing to the Bragg peak and high biological effects of the beam. However, the beam loss due to the nuclear fragmentation reactions in the patient body causes modification in the dose calculation of therapy. Although there has been several studies for the evaluation of fragmentation reactions with detectors of limited geometrical acceptances, they are not enough to cover the entire events. We established a method to identify the fragmentation reactions induced by the carbon beam by using a total-absorption-type target detector that plays the role of human phantom, as well as a system of plastic scintillation counters surrounding the phantom. We carried out measurements for the C beam with the detector system. The contributions from fragmentation reactions increase as a function of incident energies. That is, a portion of the energy of the C beam is lost as fragment particles in phantom, and leads to decrease of beam purity in consequence. Our study shows that fragmentation reactions in carbon therapy must be carefully taken into account in the treatment planning, and our data is indispensable for estimation of fragmentation reactions generated by carbon beam.