A rod lens made of a transparent cylindrical medium having a refractive index distribution in the radial direction is used as a graded index lens. An object surface and an image surface are arranged on both sides of the rod lens. A first surface of the lens is a convex spherical surface, and a second surface of the lens is a planar surface, and the lens is arranged so that the convex spherical surface faces the object surface, and the planar surface faces an image surface. The equations1.0.ltoreq.R/r.sub.0 .ltoreq.1.6, andR/f.ltoreq.1.2are satisfied, wherein r.sub.0 is a radius of a portion of the rod lens that operates as a lens, measured from an optical axis of the lens, R is a curvature radius of the convex spherical surface, and f is a focal length of the rod lens. Thus, an object lens system with a large NA, that is smaller and lighter, while keeping the number of parts to a minimum, can be provided.