1. An image processing apparatus for reducing noise in an X-ray image obtained from a digital X-ray detector, the image processing apparatus comprising: a memory storing a program; and one or more processors which, by executing the program, function as: a processing unit configured to execute low-pass filtering on the X-ray image in a specific direction and high-pass filtering on the X-ray image in a direction perpendicular to the specific direction to obtain filtered image data, and to extract linear noise in the specific direction from the filtered image data; and wherein the processing unit subtracts the extracted linear noise from the X-ray image, wherein the processing unit extracts the linear noise for segmented imaging regions obtained by segmenting the filtered image data.