A guard-ring electrode can be used at a suitable position for each of a plurality of imaging regions. Improvement in image quality and reduction in unnecessary radiation exposure, such as reduction in artifacts or noise, improvement in SNR, and expansion in dynamic range, are realized by reducing the cross-talk or the inflow of electrical charges from ineffective regions. Based on a radiation detector having a plurality of pixel electrodes 117 and a common electrode that sandwich a photoelectric conversion layer, an interelement guard-ring electrode 111 is adjacently disposed between the pixel electrodes 117 of the radiation detecting element, and the interelement guard-ring electrode 111 is switched between an electrically open-circuit state and a ground-potential connection state, so as to change the area in which the radiation detecting element detects radiation.