Provided is a technique for X-ray reflection, such as an X-ray reflecting mirror, capable of achieving a high degree of smoothness of a reflecting surface, high focusing (reflecting) performance, stability in a curved surface shape, and a reduction in overall weight. A silicon plate (silicon wafer) is subjected to thermal plastic deformation to form an X-ray reflecting mirror having a reflecting surface with a stable curved surface shape. The silicon wafer can be deformed to any shape by applying a pressure thereto in a hydrogen atmosphere at a high temperature of about 1300° C. The silicon plate may be simultaneously subjected to hydrogen annealing to further reduce roughness of a silicon surface to thereby provide enhanced reflectance.