A radiation image detector includes a radiation image detector main-body having radiation detection devices for detecting radiation that are at least linearly arranged, a switching output unit having a multiplicity of switch devices, and a dummy circuit unit connected to a switch device that is switched last among the multiplicity of switch devices. The switching output unit sequentially switches columns of the radiation detection devices by the switch devices and sequentially outputs signals detected by the radiation detection devices line by line. Further, after signals are output from the radiation detection devices in a predetermined line, the dummy circuit unit outputs a signal having a magnitude that does not substantially influence signals that will be read out from the radiation detection devices in the following line of the predetermined line.