An image information detecting apparatus comprises a light source comprising a light emitting surface, an image bearing member, a light receiving unit comprising a light receiving surface, an illumination optical system for causing the light emitting surface and the image bearing member to have a conjugate relationship and illuminating a pattern for detecting the overlap positional deviation with a beam; and a light receiving optical system for causing the image bearing member and the light receiving surface to have a conjugate relationship and guiding the beam for detecting the overlap positional deviation to the light receiving unit, wherein a straight line portion of a conjugate image of the light emitting surface on the image bearing member and a straight line portion of a conjugate image of the light receiving surface on the image bearing member are parallel with each other.