In a radiation detector, electrodes are provided on both sides of a photoconductive layer for recording. When the photoconductive layer for recording is irradiated with radiation during application of a predetermined bias voltage between the electrodes, electric charges are generated within the photoconductive layer for recording. Then, the generated electric charges are detected as an electric signal by the radiation detector. As the material for the photoconductive layer for recording, amorphous selenium having a coordination number of 1.95.+-.0.02 is used.