A light scanning unit (140) includes: a light source (11) generating and irradiating at least one beam corresponding to an image signal; a beam deflector (20) deflecting and scanning the beam irradiated by the light source (11); and an f-θ lens (30) forming an image from the beam deflected by the beam deflector (20) onto a surface to be scanned, the f-θ lens (30) being provided as one lens and satisfies the following equation (2): - 0.2 SAG ⁢ 1 + SAG ⁢ 2 d 2 0.2
where SAG 1 is Z value of an incident surface of the f-θ lens (30), which faces the beam deflector (20), SAG 2 is Z value of the exit surface of the f-θ lens (30), which faces the surface to be scanned, based on an XYZ coordinate system in which a main scanning plane is a Y-Z plane and a subscanning plane is an X-Z plane, and d 2 is a center thickness of the f-θ lens (30).