1. A flat panel detector, having a plurality of pixels in a matrix, for detecting an X-ray image, the detector comprising: first filtering means for filtering out a high frequency in a column direction of a first X-ray image obtained through at least a portion of the pixels from the first X-ray image; first subtracting means for subtracting the image obtained by the first filtering means from the first X-ray image; second filtering means for filtering out a high frequency in a row direction of the image obtained by the first subtracting means from the image obtained by the first subtracting means; and second subtracting means for subtracting the image obtained by the second filtering means from the first X-ray image.