To provide an optical device that reduces variations of beam diameters and reduces the size of the beam diameter.

A light emitting element group is within an area of rod lenses when viewing in the direction of the optical axis of the rod lens array, exists within a range from the center line of the rod lenses in a main scanning direction up to the center line of the rod lenses in the main scanning direction, and light emitting elements in the light emitting element group are arranged so as to come close to the center line of the lens array toward the center from the edge of the light emitting element group in the main scanning direction.