An optical apparatus that measures a polarization dependent characteristic of a measured object includes a light source for emitting non-linearly polarized light in an extreme ultraviolet region or an X-ray region, and a rotary polarizer for reflecting the light emitted from the light source, the polarizer including a set of mirrors repeating three or more reflections and being arranged such that an optical axis of incident light and that of outgoing light are aligned with the same straight line.