1. A method for detecting information relating to a pattern on an object to be inspected comprising the steps of: focusing and irradiating an annular-looped diffusion illumination light formed with a plurality of virtual spot light source onto a pattern on the object to be inspected through a pupil of an objective lens; receiving an image of the pattern of the inspected object by focusing a first or second order diffraction light including a 0th order diffraction light which is reflected from the pattern on the inspected object by the focused and irradiated annular-looped diffusion illumination light and entered into the pupil of the objective lens; and converting the received image of the pattern of the inspected object to image signals of the pattern for obtaining information relating to the pattern.