Provided is an optical design method for an x-ray focusing system that is novel and can focus an entire flux of light while radiating x-rays having an extremely small angle of divergence at the entire surface of a rotating mirror. The present invention comprises: a step for determining the shape of the rotating mirror (3) provided with a reflective surface formed by one rotation around the optical axis (OA) of a one-dimensional profile comprising an oval or a section combining an oval and a hyperbola such that the upstream-side focal point (F1) is set at a position deviating from the optical axis (OA) and has a focal point (F) that is the focusing point of the x-ray focusing system at the downstream side; and a step for determining, as a collection of coordinates (P) of the inflection points of the light rays in reverse ray tracing, the shape of the reflecting surface of a ring focusing mirror (4) provided with the function of focusing to a focusing ring (R) and reflecting/broadening the x-ray beam (5) radiating from the x-ray source (O) under the constraint of the light path length being constant by performing reverse ray tracing from the focusing point to the x-ray source (O) through the focusing ring (R) produced by the path of the focal point (F1) at the upstream side.
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