An X-ray waveguide showing a small propagation loss and having a waveguide mode with its phase controlled is provided. The X-ray waveguide including: a core for guiding an X-ray in a wavelength band that a real part of the refractive index of a material is 1 or less; and a cladding for confining the X-ray in the core, in which: the X-ray is confined in the core by total reflection at a interface between the core and the cladding; in the core multiple materials having different real parts of the refractive index are periodically arranged; and a waveguide mode of the X-ray waveguide is such that the number of antinodes or nodes of an electric field intensity distribution or a magnetic field intensity distribution of the X-ray coincides with the number of periods of the periodic structure in a direction perpendicular to a waveguiding direction of the X-ray in the core.