Abstract: A lens system for use in a light beam scanner for deflecting a monochromatic or quasi-monochromatic light beam, the lens system being disposed between a deflection point about which the light beam is deflected and a surface to be scanned in the light beam scanner. The lens system comprises an f.theta. lens and a corrective lens. The f.theta. lens comprises two positive-meniscus lenses with their concave surfaces directed toward the deflection point, the f.theta. lens having an effective deflection angle of at least 50 degrees. The corrective lens cooperates with the f.theta. lens in constituting a surface irregularity correcting optical system and is disposed between the f.theta. lens and the surface to be scanned. The corrective lens comprises a toroidal lens capable of correcting surface irregularity and curvature of field.