An X-ray condensing method and its device are provided with an X-ray mirror that has a wavefront adjustable function to finely adjust a wavefront of a reflecting X-ray, measure an X-ray intensity distribution in the vicinity of a focus, measure an X-ray intensity distribution in the vicinity of the X-ray mirror or use a known X-ray intensity distribution of an incident X-ray, calculate a complex amplitude distribution at the reflective surface by using a phase restoration method from the X-ray intensity distribution in the vicinity of the focus and the X-ray intensity distribution in the vicinity of the reflective surface, calculate a wavefront aberration of an X-ray condensing optical system from the complex amplitude distribution, and control the reflective surface of the X-ray mirror with the wavefront adjustable function so that the wavefront aberration is minimized.