1. A radiation image detecting device comprising: a panel unit having an image capturing field for imaging a radiographic image of an object by receiving radiation emitted from a radiation source; a plurality of pixels arranged in said panel unit in a two-dimensional array having a plurality of pixel-rows and a pixel-column, each of said pixels producing and accumulating electric charge; a plurality of scan lines provided in said panel unit on a pixel-row basis, for making said pixel-row having said pixels from which said electric charge is to be read out in an ON state; a signal line provided in said panel unit on a pixel-column basis, for reading out said electric charge from said pixels on a pixel-column basis; a controller for controlling said panel unit to perform three types of operations of a pre-emission readout operation, an accumulation operation, and an image readout operation, wherein in said pre-emission readout operation, a plurality of adjoining said pixel-rows are set as a binning pixel-row, and binning readout of said electric charge is performed on a binning pixel-row basis sequentially from a first binning pixel-row to a last binning pixel-row and repeated from said first binning pixel-row upon reaching said last binning pixel-row, in order to obtain reference line images each having a pixel value being a sum of said electric charge of a plurality of said pixels in a same pixel-column, said accumulation operation is performed instead of said pre-emission readout operation, in a case where said radiation source starts emitting said radiation while said binning readout of said electric charge is repeated on a binning pixel-row basis, for accumulating said electric charge in said pixels in accordance with said radiation, and said image readout operation is started after completion of emission of said radiation from said radiation source, for reading out said electric charge from said pixels on a pixel-row basis and converting said electric charge to pixel values for forming said radiographic image; a reference line image record controller for obtaining said reference line images of a plurality of said binning pixel-rows by sequentially recording said reference line images to a memory whenever performing said binning readout; an emission start judging unit for judging start of emission of said radiation from said radiation source; a correction image generator for producing a correction image used for correcting a strip-shape line defect occurring in a pixel-row direction of said radiographic image due to a time delay between start of emission of said radiation and a judgment of the start of emission, by scaling up an image size of said reference line images of said plurality of binning pixel-rows in a pixel-column direction and correcting said pixel values; and a line defect corrector for correcting said line defect by adding said correction image to said radiographic image.