A radiation image detecting device, comprising: a plurality of pixels arranged in two dimensions, each of said pixels being adapted to accumulate signal charge corresponding to an amount of radiation from a radiation source; a plurality of switching elements each of which is provided to correspond to each of said pixels, each of said switching element making the corresponding pixel perform a charge accumulation operation of said signal charge and a readout operation of said signal charge; a signal processing circuit adapted to convert said signal charge read out from said pixels into an electric signal and outputting said electric signal; an irradiation detector adapted to detect whether irradiation of said radiation is started or not based on an electrical change on said pixels; and a controller adapted to stop supplying electric power to said signal processing circuit until before the start of the radiation irradiation is detected by said irradiation detector and to start supplying the electric power to said signal processing circuit when the start of the radiation irradiation is detected by said irradiation detector, characterized in that said controller is adapted to turn on all of said switching elements until before the irradiation of the radiation is detected by said irradiation detector, and adapted to turn off all of said switching elements to be shifted to said charge accumulation operation when the irradiation of the radiation is detected by said irradiation detector, said controller being adapted to start supplying the electric power to said signal processing circuit after turning off all of said switching elements. a plurality of pixels arranged in two dimensions, each of said pixels being adapted to accumulate signal charge corresponding to an amount of radiation from a radiation source; a plurality of switching elements each of which is provided to correspond to each of said pixels, each of said switching element making the corresponding pixel perform a charge accumulation operation of said signal charge and a readout operation of said signal charge; a signal processing circuit adapted to convert said signal charge read out from said pixels into an electric signal and outputting said electric signal; an irradiation detector adapted to detect whether irradiation of said radiation is started or not based on an electrical change on said pixels; and a controller adapted to stop supplying electric power to said signal processing circuit until before the start of the radiation irradiation is detected by said irradiation detector and to start supplying the electric power to said signal processing circuit when the start of the radiation irradiation is detected by said irradiation detector, characterized in that said controller is adapted to turn on all of said switching elements until before the irradiation of the radiation is detected by said irradiation detector, and adapted to turn off all of said switching elements to be shifted to said charge accumulation operation when the irradiation of the radiation is detected by said irradiation detector, said controller being adapted to start supplying the electric power to said signal processing circuit after turning off all of said switching elements.