An optical scanner is configured such that an anamorphic condensing lens in an incident optical system has a diffractive lens structure at least in one lens surface thereof, and a length of an optical path increased by the diffractive lens structure φ [rad] is defined by an equation below by a function of height h from an optical axis: φ(h)=M(P2·h2+P4·h4+ . . . ), where Pn is a coefficient of an nth-order term of the height h (n is an even number), and M is a diffraction order, that the lens satisfies the following relations: −216≦P2≦−49, 1100≦P4·(hm max)4/(fm·NAm4)≦3800, and 10≦fm≦35, where hmmax [mm] is an effective diameter in the main scanning direction, fm [mm] is a focal length in the main scanning direction, and NAm is a numerical aperture in the main scanning direction, and that a wavefront aberration WFE1 [λrms] in a first wavelength λ1 [nm] satisfies the following relation: WFE1≦0.01.