1. An X-ray image processing apparatus comprising: processor circuitry: configured to perform frequency analysis on a first X-ray image based on an output signal from an X-ray detector influenced by an action of an electromagnetic field and specify, as a noise characteristic, a specific frequency axis and a specific frequency range where noise components originating from the action of the electromagnetic field on the X-ray detector and contained in the first X-ray image exist; and configured to decide a filter characteristic for reducing the noise components contained in the first X-ray image based on the noise characteristic in the first X-ray image; and a filter configured to perform filter processing for at least one of the first X-ray image and a second X-ray image generated after the first X-ray image, based on the filter characteristic, wherein the processor circuitry is further configured to generate from the first X-ray image a two-dimensional data in a two-dimensional frequency space constituted by a horizontal frequency axis and a vertical frequency axis, specify the specific frequency axis based on a pixel value change in a first range on the horizontal frequency axis and a pixel value change in a second range on the vertical frequency axis, and specify the specific frequency range by performing threshold processing for a pixel value change along the specific frequency axis.