A scanning optical device includes a light source that emits a light beam; a deflector for deflecting and scanning the light beam from the light source; a scanning lens for converging the deflected light beam onto an image surface; and a mechanism for adjusting the relative position of at least one lens element of the scanning lens with respect to other lens elements of the scanning lens in an auxiliary scanning direction. Preferably, the adjustable lens element of the scanning lens be provided with an anamorphic surface having relatively larger power in the auxiliary scanning direction than in the main scanning direction. Alternatively, the above-mentioned at least one lens element may have the largest absolute value of refractive power of the lenses in the scanning lens. Further alternatively, the above-mentioned at least one lens element may be provided with a surface having the smallest absolute value of radius of curvature of all surfaces in the scanning lens.