A projection optical system according to the present invention whose image side numerical aperture is greater than or equal to 0.75, and which forms an image of a first object upon a second object using light of a predetermined wavelength less than or equal to 300 nm, comprises: a first lens group G<HIL><PDAT>1 < / BOLD><PDAT>of positive refractive power; a second lens group G<HIL><PDAT>2 < / BOLD><PDAT>of negative refractive power; a third lens group G<HIL><PDAT>3 < / BOLD><PDAT>of positive refractive power; and a fourth lens group G<HIL><PDAT>4 < / BOLD><PDAT>of positive refractive power, and: the first lens group G<HIL><PDAT>1, < / BOLD><PDAT>the second lens group G<HIL><PDAT>2, < / BOLD><PDAT>the third lens group G<HIL><PDAT>3 < / BOLD><PDAT>and the fourth lens group G<HIL><PDAT>4 < / BOLD><PDAT>are arranged in order from a side of the first object; and a distance D in mm along an optical axis between an optical surface of the fourth lens group G<HIL><PDAT>4 < / BOLD><PDAT>closest to the second object, and the second object, satisfies a condition of 0.1<D<5.