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 processing unit configured to apply, to the X-ray image, an averaging process of values of pixels of the X-ray image lined in a specific direction to obtain a data set including a plurality of items of data lined in the specific direction, and to extract linear noise in the specific direction from the data set; 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 data set, the segmented imaging regions obtained by segmenting the data set in the specific direction.