In an X-ray diffraction method using the parallel beam method, an X-ray parallel beam is incident on a sample, and diffracted X-rays from the sample are reflected at a mirror and thereafter detected by an X-ray detector. The reflective surface of the mirror has a shape of an equiangular spiral that has a center located on the surface of the sample. A crystal lattice plane that causes reflection is parallel to the reflective surface at any point on the reflective surface. The X-ray detector is one-dimensional position sensitive in a plane parallel to the diffraction plane. A relative positional relationship between the mirror and the X-ray detector is determined so that reflected X-rays from different points on the reflective surface of the mirror reach different points on the X-ray detector respectively. This X-ray diffraction method is superior in angular resolution, and is small in X-ray intensity reduction, and is simple in structure.