A scanning optical system includes first and a second lens elements, at least one of which is a plastic lens element. Each surface of the plastic lens element is an aspherical surface that is configured such that a shape in a main scanning plane is defined as a function of a distance, in the main scanning direction, from a surface reference axis thereof, and that a curvature in an auxiliary scanning plane which is perpendicular to the main scanning plane is defined as another function of a distance, in the main scanning direction, from the surface reference axis. At least one of the two surfaces of the plastic lens element are arranged such that the surface reference axis thereof is decentered from an axis of the beam in the auxiliary scanning direction.