The invention is to provide a light scanning unit designed to suppress the quantity of torsion of a light beam caused by an inclination angle of a cylindrical lens disposed between a light source and an optical deflecting element so as to suppress increase in diameter of the beam imaged on a to-be-scanned surface. The object is achieved by the light scanning unit in which a collimator lens, an aperture, a cylindrical lens, a spherical lens, a rotary polygon mirror, an F.theta. lens and a to-be-scanned surface are disposed in an optical path between the light source and the optical deflecting element in order of increasing distance from the light source. The light scanning unit is designed to satisfy an expression of M.sub.t.ltoreq.D/.delta. when .delta. designates a diameter of the aperture, D designates a diffraction diameter of each beam focused on the to-be-scanned surface and scanning the to-be-scanned surface, .gamma. designates a rotation angle with a generatrix of the cylindrical lens, and M.sub.t designates a conjugate magnification.