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 unit configured to execute high-pass filtering in a column direction on pixel values of the original image data to obtain first image data; a horizontal noise extracting unit configured to extract a horizontal noise component which decreases an edge component of an object in the original image data from the first image data by decreasing an absolute value of the first image data in a region in which changes in pixel value in a same pixel row of the first image data is greater than or equal to a predetermined value; and a processing unit configured to subtract the horizontal noise component from the original image data to obtain processed image data.