A light scanning unit may include a light source to emit a light beam and a beam deflector to reflect and scan the light beam emitted from the light source in a main scanning direction. The light scanning unit may also include a beam detecting sensor to detect a horizontal synchronization signal by receiving a part of the light beam and may include a circuit board disposed in part of a trajectory of the light beam such that the light beam is incident on a first surface of the circuit board. The circuit board may include a through-hole formed in the circuit board in the trajectory of the light beam. The beam detecting sensor may be mounted to a second surface of the circuit board which is opposite to the first surface of the circuit board. The light beam may be incident on the beam detecting sensor via the through-hole.