An optical unit includes an optical module that includes a light emitting element array in which a plurality of light emitting elements are arranged, a lens array disposed facing the light emitting element array on an optical path of light emitted from the plurality of light emitting elements, and a fixing part configured to fix the light emitting element array to the lens array. The optical unit also includes a member configured to have a larger coefficient of thermal expansion than a coefficient of thermal expansion of the light emitting element array and a fastening part configured to fasten the optical module on a surface of the member. The fastening part is configured to fasten the optical module so as to enable expanding and contracting in a plane parallel to the surface of the member.