Of lenses constituting an f.multidot..theta. lens, with respect to the lens L2 having a greatest difference between absolute values of surface power at both surfaces thereof, a convex surface having a greater absolute value of surface power is made to abut through line contact against columnar abutment members projecting from the optical housing 10 perpendicularly to the scanning direction. In addition, flat portions provided at both ends of the lens in the scanning direction thereof are supported by supporting members, and the lens is affixed to the abutment members and by means of leaf springs.