A front group consists of two concave lenses. A rear group consists of: a positive lens in which the object-side surface has a larger radius of curvature; and a cemented lens configured by a positive lens in which the object-side surface has a larger radius of curvature, and a negative meniscus lens, the cemented lens having being positive as a whole. An objective lens satisfies the following conditional expressions: |d.sub.x/f.sub.F|.gtoreq.3.0 (1) (f/f.sub.3).times..nu..sub.3<23 (2) f.sup.2.times.(.nu..sub.5-.nu..sub.4)/{R.sub.A.times.(Bf+d.sub.5/n.su- b.5)}>7 (3) where d.sub.x is the distance between the groups, f, f.sub.F, and f.sub.3 are the focal lengths of the objective lens, the front group, and the lens, respectively, .nu..sub.3, .nu..sub.4, and .nu..sub.5 are the Abbe numbers of the lenses, R.sub.A is the radius of curvature of the cementing surface between the lenses, B.sub.f is the back focus, and d.sub.5 and n.sub.5 are the center thickness and refractive index with respect to the d-line of the lens.