PURPOSE: To obtain a small-sized lightweight lens which has large numerical aperture and operation distance by allowing the lens system of four-group constitution which consists of a biconvex or convex meniscus lens, concave meniscus lens, biconvex lens, and cemented lens of a biconvex and a biconcave lens successively from a light source side to meet specific requirements.

CONSTITUTION: The lens system of four-group constitution consists of the biconvex or convex meniscus lens I , concave meniscus lens II, biconvex lens III, and cemented lens IV of the biconvex and biconcave lenses successively from the light source side and meets specific requirements, i.e. 0.55<r5/r6<1.05 and 0.95< r3/d3<1.75, where r5 and r6 are the radii of curvature of the surfaces of the group III on the sides of the light source and a disk 6, r3 is the radius of curvature of the light-source side surface of the 2nd group II, and d3 is the thickness of the 2nd group lens II. Consequently, the small-sized, lightweight lens which has large numerical aperture and sufficient operation distance is obtained.