Radiation detector made of synthetic diamond has many advantages as high resolution and pulse mode radiation detector. Synthetic diamond detector has been investigated its characteristics with electric tests and radiation sources in laboratories. In order to investigate more detail of its character, high energy heavy ion beams from HIMAC were irradiated to the synthetic diamond detector. Because these beams can penetrate the diamond detector and give it much deposit energy, characteristic at deeper part of synthetic diamond detector can be investigated. Using heavy ion beams like Iron 500 MeV/u, Silicon 800 MeV and so on, saturation curve for bias voltage, polarization effect, which is a phenomenon of decreasing of output signals and energy resolution of a diamond detector for deposit energy were investigated.