An X-ray waveguide, for guiding X-rays having a wavelength of 1 pm or more and 100 nm or less, includes: a core and a cladding. The core has a periodic structure composed of a plurality of materials each having a different real part of refractive index in the direction perpendicular to the waveguiding direction. A planarizing layer is disposed between the core and the cladding. The critical angle for total reflection of the X-rays at the interface between the planarizing layer and the cladding is larger than the Bragg angle of the periodic structure of the core.