In order to provide a radiation detector capable of implementing measurements with a good energy resolution and a high detection efficiency over a broad energy range using a single detector, in the present invention, a radiation detecting element composed of Si semiconductor and the radiation detecting element composed of CdZnTe or CdTe semiconductor are lined up as two layers longitudinally. The radiation detecting element composed of Si semiconductor is taken as a first layer at the side of incidence of the radiation and the radiation detecting element composed of CdZnTe or CdTe semiconductor is taken as a second layer.