An optical writing unit includes in a housing a light source projecting a light beam, a deflector to deflect the light beam projected from the light source, a focusing lens, and a lens adjusting device. The focusing lens is long in a main scanning direction and focuses the light beam onto a scan target. The lens adjusting device rotates the focusing lens about a substantially center portion thereof as a rotation fulcrum on a plane perpendicular to an optical path of the light beam so as to adjust the position of the light beam illuminating the scan target. The focusing lens has an asymmetric cross-section relative to a center line of the focusing lens in a direction of the optical path in a sub-scanning plane perpendicular to the main scanning direction, and the center of gravity of the focusing lens is offset from the center line toward the rotation fulcrum.