In the inspection apparatus of this projection optical system, a folding glass member, comprising a flat surface part and a reflecting spherical surface part opposing to the flat surface part, is disposed on the image plane side of the projection optical system so that the flat surface part opposes to the projection optical system. Further, in a state wherein a liquid is supplied between the projection optical system and the folding glass member, a measuring beam emitted from an interferometer unit enters the projection optical system; the measuring beam that transmitted through the projection optical system and the liquid, and entered the folding glass member is reflected by the reflecting spherical surface part, and once again passes through the liquid and the projection optical system; and the interference fringes obtained from the measuring beam and the reference beam generated within the interferometer unit are detected.