A detection panel has a plurality of pixels for accumulating electric charge by receiving X-rays, and a plurality of detection pixels for detecting an X-ray dose in an imaging surface. The detection pixels are disposed periodically with leaving space. A grid, which has X-ray absorbing portions and X-ray transmitting portions alternately and periodically arranged in a first direction, is disposed in a position opposed to the imaging surface. Since an arrangement period of the detection pixels in the first direction is different from an arrangement period of the X-ray absorbing portions, an output value of each detection pixel is distributed and hence the average of the output values has a reduced variation range.