To excellently compensate off-axis aberration by composing an objective lens system of a first and a second lenses whose convex surfaces confront a long conjugate distance side and imparting aspherical surfaces to the lenses.

This system is composed, in order from the long conjugate distance side, of a first lens L1 whose convex surface confronts the long conjugate distance side and a second lens L2 whose convex surface confronts the long conjugate distance side. The first lens L1 and the second lens L2 are composed so as to have aspherical surfaces. An aspherical surface in the first lens L1 confronts the long conjugate distance side and an aspherical surface in the second lens L2 is provided on the convex surface on the long conjugate distance side. In this case, the system is composed so that a light ray is strongly diffracted on the convex surfaces on the sides of the long conjugate distance of the first and the second lenses L1, L2 and aberration is liable to be generated. Thus, the generation of aberration is suppressed by the aspherical surface.