A radiation detector 100 has a plurality of single-photon-counting imaging cells, including an imaging cell 1 configured to generate a detection signal in accordance with the intensity of radiation, a digitization circuit 5 configured to digitize the detection signal, a data read structure 11 configured to count the digitized detection signal and to keep the result as a data bit, and a send-out register 13 configured to control the input of a predetermined data bit output from the data read structure 11 by a data shift in accordance with a first clock Pck, to keep the input data bit as a result of the control, and to send out the kept data bit by a data shift in accordance with a second clock Sck.