To provide a reflective/refractive optical system having a sufficiently large numerical aperture and a working distance on the image side, an excellent image forming characteristic, and its object plane being parallel with its image plane.

A light beam from an object plane forms the reduced image of the object plane on an image plane being almost parallel with the object plane through a first lens group G1 having a positive refractive power, an optical path deflecting means FM having a plane for deflecting the optical path, a second lens group G2 having a negative refractive power, a concave reflection mirror having an enlarging magnification, a beam splitter BS and a third lens group G3 having a positive refractive power. The position of a principal point of the third lens group G3 is located on the image side farther than the farthermost surface on the object side of the third lens group G3.