PURPOSE: To realize a small size and low cost of an objective lens which uses a semiconductor laser as a light source, by satisfying the necessary conditions with a 5-lens structure of the 1st biconvex lens, the 2nd negative meniscus lens with its concave face turned toward a photodetector, the 3rd biconvex lens and the 4th biconvex-biconcave junction lens respectively.

CONSTITUTION: A lens group consists of the 1st biconvex lens 1, the 2nd negative meniscus lens 2 with its concave face turned toward a photodetector, the 3rd biconvex lens 3 and the 4th biconvex-biconcave junction lens 4. The conditional equations (1)W(4) are satisfied, where the radius of curvature on the lens surface is referred to as (r), the air space is (d), the focal distance is (f), etc. In such constitution, an objective lens that assures a 6.5mm outer diameter, a 10mm total length, about 0.8g weight, 0.57NA and a ≥1.4mm working distance is obtained to be suitably applied to the recording and reproducing purposes of a video disk using a semiconductor laser for its light source.