An optical writing unit includes a light-emitting-element array in which a plurality of light emitting elements is arranged and an optical system that guides light flux emitted from the light emitting elements as a light spot. A result of comparison of a property value of the light emitting elements at a predetermined threshold in an exposure intensity distribution of the light emitting elements is within a preset range over an entire effective image area. An emission condition of the light emitting elements is set such that a fluctuation of amounts of exposure of the light emitting elements or a result of comparison of the amounts of exposure does not exceed a preset range over the entire effective image area.