A light scanning unit includes a light source unit emitting a light beam according to an image signal, a light deflector scanning the light beam that is deflectively emitted by the light source unit, a housing having a side portion where the light source unit is provided and a base surface on which the light deflector is provided, and a deformation prevention member connecting opposite sides of the housing across an upper side of the base surface. The light scanning unit is employed by an image forming apparatus.