a collimator lens system for collimating diverging light bundle emitted from a light source into a parallel light bundle includes first to third lens elements arranged in this order from the side of the parallel light bundle . the first lens to third elements are a positive double - convex lens , a negative lens concave to the side of the parallel light bundle and a positive lens convex to the side of the parallel light bundle , respectively . the following formulae to are satisfied 0 . 8 & lt ; f . sub . 1 / f . sub . 3 & lt ; 2 . 0 1 . 5 & lt ;}/}& lt ; 2 . 7 0 . 8 & lt ; r . sub . 5 /}& lt ; 1 . 4 0 . 8 & lt ;- r . sub . 1 / r . sub . 2 & lt ; 2 . 4 0 . 5 & lt ;- f . sub . 2 / f & lt ; 6 . 0 wherein f represents the overall focal length of the collimator lens system , f i represents the focal length of i - th lens element , i standing for 1 , 2 or 3 , r i represents the radius of curvature of i - th refracting surface as numbered from the side of the parallel light bundle , i standing for 1 , 2 , 3 , 4 , 5 or 6 and n i represents the refractive index of i - th lens , i standing for 1 , 2 or 3 .