A tandem laser scanning unit (LSU) is provided. The tandem LSU includes a light source, a beam deflector, and an optical scanning lens. The beam deflector rotates on a light-emitting path of the light source to simultaneously deflect different light beams obliquely incident at a predetermined angle to each other within a sub-scanning plane substantially parallel to the rotational axis of the beamdeflector. The optical scanning lens allows the light beams deflected by the beam deflector to be condensed on corresponding photosensitive drums. The optical scanning lens includes at least one sub-scanning cross-section having an aspherical surface to reduce the curvature of a scanning line. Using the tandem LSU, the distortion and curvature of the scanning line are corrected while providing anoblique incidence optical system having advantages in terms of manufacturing costs and driving efficiency.