A laser scanning unit includes a light source for emitting a plurality of laser beams, a rotating polygon mirror for deflecting the plurality of laser beams in a predetermined direction, an f.theta. lens for transmitting the deflected laser beams to be scanned onto the surfaces of a plurality of photosensitive drums, and a beam splitter disposed at a rear end of the f.theta. lens for splitting the laser beams. The laser scanning unit may be used for a tandem image forming apparatus.