1. An apparatus which detects a defect of a repeated pattern formed on a substrate surface of a substrate, the apparatus comprising: a stage on which the substrate is placed; a light source; an objective lens; an illumination optical system for epi-illumination of the substrate surface with linearly polarized light, from the light source transmitted through the objective lens; an analyzer that is defined by a transmission axis; an imaging device which takes a pupil image of the objective lens that is formed with the light of illumination reflected from the substrate surface and transmitted through the objective lens and the analyzer, the transmission axis of the analyzer being set to satisfy a cross-nichols condition along with a direction of vibration of the linearly polarized light; and a detecting unit which compares the pupil image to a pupil image of a substrate surface that does not have a defect to detect.