It provides an optical design method of a new X-ray condensing system capable of condensing the entire flux while irradiating X-rays having a very small divergence angle to the entire surface of the rotating mirror. A one-dimensional profile consisting of an ellipse or a combination of elliptic and hyperbolic curves with the focal point (F) serving as the condensing point of the X-ray condensing system on the downstream side and the focal point (F1) on the upstream side set off the optical axis (OA). The step of determining the shape of the rotating mirror 3 having a reflective surface formed by rotating one rotation around the optical axis OA, and the condensing ring R created by the trajectory of the focus F1 on the upstream side After passing through, and tracing back light from the condensing point to the X-ray source (O), under the constraint that the optical path length is constant, the X-ray beam (5) irradiated from the X-ray source (O) is reflected and widened to collect light on the condensing ring (R). It consists of the step of determining the shape of the reflection surface of the ring condensing mirror 4 with a function as a set of coordinates P of the bending points of the light beams for backlight tracking.
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