A development device includes an imaging lens and light shielding walls. The imaging lens includes a body part having a constant diameter and both end parts arranged at both ends in an optical axial direction having diameters larger than the body part to capture a reflected light from a document onto an imaging part. The light shielding walls has a space with an interval larger than the diameter of the body part and smaller than the diameters of the both end parts, allowing the body part of the imaging lens to be arranged in the space, and shielding a light in the optical axial direction.