1. An image processing apparatus comprising: a flat panel detector including a reading circuit configured to convert X-rays to original image data; a filtering device configured to execute high-pass filtering in a column direction on pixel values of the original image data to obtain first image data; and a processing unit configured to subtract a value obtained by converting the pixel values of the first image data in accordance with an absolute value of a statistic calculated from the pixel values in a same pixel row of the first image data from values of corresponding pixels of the original image data to obtain processed image data.