An optical element for X-ray includes a substrate, a first multilayer film having a reflection property with respect to light in a soft X-ray wavelength range, and a second multilayer film, disposed between the substrate and the first multilayer film, for reducing film stress of the first multilayer film. The second multilayer film has a periodic structure having a unit period film thickness which is 90% or more and less than 110% of a two or more integral multiple of 7 nm.