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: at least one processor and memory coupled to each other and cooperating to act as a processing unit configured to apply, to the X-ray image, a low-pass filter process in a specific direction to obtain a filtered image including pixels in the specific direction, and to extract linear noise in the specific direction from the filtered image; and a reduction unit configured to reduce noise of the X-ray image based on the extracted linear noise, wherein the processing unit is configured to perform a process for extracting the linear noise for each of segmented imaging regions of the X-ray image, the segmented imaging regions obtained by segmenting the X-ray image in the specific direction.