Patent ID: 8760483

Claim:
An exposure device comprising: a light source device including a plurality of light emitting devices arrayed in a one-dimensional or a two-dimensional array, to project light; a light source holding member to hold the light source device in place, the light source holding member being made of metal; an optical device to focus the light projected from the light source device onto an image bearing member; an optical device holding member to hold the optical device so as to maintain a predetermined gap between the optical device and the light source device on the light source holding member; a positioning member fixed to a side of the light source holding member in a longitudinal direction of the light source holding member to support the light source holding member above the image bearing member so as to maintain a predetermined gap between the image bearing member and the light source device on the light source holding member, the position at which the positioning member supports the light source holding member being behind a light projection point of the light source device in a direction opposite the image bearing member; and a contact member that separates the positioning member and the image bearing member, wherein the following relation is satisfied: k 1< k 2 where k 1 is a linear expansion coefficient of the light source holding member and k 2 is a linear expansion coefficient of the positioning member.