PROBLEM TO BE SOLVED: To provide an infrared-ray catadioptric system that is resistant to a temperature change, while allowing for an aperture matching configuration.SOLUTION: An infrared-ray catadioptric system 1 comprises, in order from an object side,: a first image formation optical system G1 that is for forming a first intermediate image; a first optical path folding mirror FM1 that is placed in the vicinity of a formation position of the first intermediate image, and is for folding a ray from the first image formation optical system G1; a second image formation optical system G2 that has a concave reflection mirror CM and at least one negative lens L21, and is for forming a second intermediate image in the vicinity of the formation position of the first intermediate image on the basis of a ray from the first intermediate image; a second optical path folding mirror FM2 that is placed in the vicinity of a formation position of the second intermediate image, and is for folding a ray from the second image formation optical system G2; and a third image formation optical system G3 that is for forming a final image on an imaging plane I on the basis of the ray from the second intermediate image. Let an absolute value of a relay image formation magnification to be taken by the third image formation optical system G3 be &bgr;, the infrared-ray catadioptric system 1 satisfies 2.0&bgr;3.3.
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