An apparatus measuring wavefront aberration of an optical system includes a first mask in an object plane and having plural openings, an illumination optical system illuminating the openings of the first mask by using light from a light source, a second mask in an image plane and having an opening allowing passage of light containing aberration of the optical system and a pinhole/slit. The apparatus takes an image of interference fringe generated by light having passed through the optical system and the pinhole/slit and the light having passed through the optical system and the opening of the second mask, calculates an evaluation value to evaluate a state of the interference fringe by using image data of the image, determines, based on the evaluation value, whether the wavefront aberration of the inspected optical system is to be calculated, and calculates the wavefront aberration of the optical system from the image data.