An object of the invention is to provide a novel optical design method for an X-ray focusing system capable of collecting all the fluxes, while applying an X-ray of a very small divergence angle to the entire surface of a rotating mirror. The method includes a step of determining the shape of a rotating mirror (3) provided with a reflection surface, the reflection surface being formed by rotating, by one turn around an optical axis (OA), a one-dimensional profile composed of an ellipse or a part of combination of the ellipse and a hyperbolic curve, the ellipse including a downstream focal point (F) serving as a light collecting point of the X-ray focusing system, and including an upstream focal point (F1) deviated from the optical axis (OA); and a step of determining the shape of a reflection surface of an annular focusing mirror (4).