It is an object of this invention to provide a light scanning device capable of suppressing scanning line bending to a low level, which is caused by the arrangement error of a single-element lens (6) serving as an imaging optical element, and an image forming apparatus using the device. In order to achieve the object, according to this invention, an imaging optical system is formed from a single-element lens (6), and the sectional shapes of the incident (6a) and exit (6b) surfaces in the main scanning direction are non-arcuated, and the power in the sub scanning direction substantially concentrates on the exit surface (6b). The non-arcuated shape (curvature) of the exit surface in the main scanning direction is so determined as to make the magnification in the sub scanning direction uniform.