To provide an X-ray waveguide which: shows a small propagation loss of an X-ray; has a waveguide mode with its phase controlled; does not deteriorate owing to oxidation; and can be easily produced, an X-ray waveguide, including: a core for guiding an X-ray in such 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 core has a one-dimensional periodic structure containing multiple materials having different real parts of the refractive index; the multiple materials include one of an organic material, a gas, and a vacuum, and an inorganic material; and the core and the cladding are formed so that the critical angle for total reflection at an interface between the core and the cladding is larger than a Bragg angle resulting from a periodicity of the one-dimensional periodic structure, is realized.