1. An image processing method of processing image data obtained from an X-ray detector having a phosphor plate which converts an X-ray into light, and an image sensor which converts light emitted from the phosphor plate into an electric signal, said method comprising: an acquiring step of acquiring a resolution distribution of the phosphor plate; an obtaining step of obtaining image data from the X-ray detector; and a filter processing step of executing filter processing by applying spatial filters such that a spatial filter is selected as the spatial filter at a pixel position based on a two-dimensional spatial filter table of indexes of resolution and indexes of dose.