To make it possible to suppress the occurrence of the color differences of spherical aberrations and the occurrence of chromatic aberrations over a wide wavelength range from a near IV region to a near IF region without increasing the number of elements of the lens group on the extreme object side by distributing the power of a first lens group and second lens group having positive power with a good ballance power.

The long-foucus objective lens is composed of the positive first lens group and the positive second lens group arranged apart therefrom. The first lens group is composed by arranging, successively from an object side, a concave-convex first lens and a negative second lens of which the concave face is directed to the object side. The second lens group is composed of a biconvex third lens and a biconcave fourth lens. The objective lens is so constituted as to satisfy the respective conditions 1.10<fI/f<2.00, θp-θn<0.02, 0.3<L/f<0.06, where (f) is the focal length of the entire system; fI is the focal length of the first lens group; θp, θn are respectively the average of the dispersion ratios of the parts expressed by (ng-nF)/(nF-nC) of the positive lenses and negative lenses and L is the spacing between the first and second lens groups.