Light is irradiated on a light-shielding pattern on an object surface side of a projection optical system, and light intensity distribution of the light having passed through the projection optical system and slits is detected while slits of an aerial image measuring unit on the image plane side of the projection optical system are moved within a plane perpendicular to the optical axis of the projection optical system, The information concerning the flare of the projection optical system is computed from the light intensity distribution, so that the influence of resist coated on a wafer used in a conventional exposing method can be eliminated, and highly accurate measurement of information concerning the flare can be realized. Further, measurement of information concerning the flare can be performed in a short time comparing to the exposing method because development process or the like of the wafer is not necessary.