Provided is a detecting apparatus including an illumination optical system for emitting an image bearing member with a light beam emitted from a light source unit, an imaging optical system including an imaging optical element for imaging an image on the surface of the image bearing member, a light receiving unit for detecting the image on the image bearing member which has been imaged by the imaging optical system, and a calculation unit for detecting image information on the image on the image bearing member from a detection signal received from the light receiving unit. In the detecting apparatus, at least one optical plane of the imaging optical element is configured to refract an imaging system principal ray after passing therethrough in a direction of farther becoming apart from the light source unit than the imaging system principal ray before passing through the optical plane.