PURPOSE: To decrease an F value and reduce the diameter of a spot and to adapt the telecentric fθ lens to a scanning optical system with a rotary polygon mirror of equal-angular-speed rotation and a surface tilt correcting mechanism for a cylindrical lens by employing five-lens constitution and satisfying specific requirements.

CONSTITUTION: This lens consists of a positive meniscus lens as a 1st lens L1 which has its concave surface on the object field side, a negative lens as a 2nd lens L2, a positive meniscus lens as a 3rd lens L3 which has its concave surface on the object field side, a positive meniscus lens as a 4th lens L4 which has its concave surface on the object field side, and a positive lens as a 5th lens L5 in order from the object field side. Further, conditions 1.7<n1, 1<r5/r6<1.1, and 1<d8<f<1.1 are satisfied, where n1 is the refractive index of the 1st lens L1, r5 and r6 the radii of curvatures of the 5th and 6th lens surfaces counted from the object-field side, d8 the air gap on the optical axis between the 4th lens L4 and 5th lens L5, and (f) the focal length of the whole system.