A light scanning unit includes a light source, a collimating unit for collimating light emitted from the light source, and a rotatory polygonal mirror for deflecting light radiated from the collimating unit. One sheet of an f-theta lens scans the light deflected by the rotatory polygonal mirror to a plane at a substantially uniform velocity to form an image on the plane and to correct a field curvature aberration in a main scanning direction. The f-theta lens may be a meniscus lens having a convex surface directed toward a deflection plane. A curvature of the f-theta lens in the main scanning direction differs from a curvature in a sub scanning direction. The f-theta lens has an aspherical shape in which a curvature in the sub scanning direction is varied continuously. A ratio of the radius of curvature of a first surface to the radius of curvature of a second surface at an optical axis is approximately at least 1.7.