A radiation detector is realized as, for example, a flat panel detector (FPD) for two-dimensionally detecting X-rays. The detector comprises a detection board and a control unit. The detection board on which a plurality of detection elements for detecting a radiation are two-dimensionally disposed in a matrix and circuits for collecting signals from the detection elements. The control unit for controlling the detection board so that the detection board is placed in a predetermined temperature state. The predetermined temperature state is defined, for example, as predetermined ranges of temperatures used during the operation and the non-operation states of the detector, respectively. This radiation detector can be mounted in a radiography system such as an X-ray radiography system.