An X-ray apparatus that creates a virtual source having a narrow energy bandwidth and enables a high-resolution X-ray diffraction measurement; a method of using the same; and an X-ray irradiation method are provided. An X-ray apparatus 100 includes a monochromator 105 that focuses a divergent X-ray beam while dispersing it and a selection part 107 that is installed in a condensing position of the condensed X-ray beam for selecting an X-ray beam having a wavelength in a specific range, allowing it to pass through, and creating a virtual source. With this arrangement, it is possible to create a virtual source having a narrow energy bandwidth at a focal point 110 and by means of the virtual source a high-resolution X-ray diffraction measurement is available. By using the X-ray apparatus 100, it is possible to sufficiently separate an X-ray beam having such an extremely narrow energy bandwidth as, for example, Kα1 ray from Kα2 ray. In addition, it is also possible to cut out part of continuous X-ray beams to create a virtual source.