From body side has negative power the 1st glass lens which (L1), to squeeze to sequence and (S), has positive power the 2nd glass lens which (L2), the 3rd glass lens (L3) consist of, the 1st glass lens (L1) most the surface of image side (2) the concave surface, the 2nd glass lens (L2) most the surface of image side (6) to be formed the aspheric surface, the 3rd glass lens (L3) most the surface of body side (7) the convex surface, with, the 2nd glass lens (L2) with the 3rd glass lens (L3) distance d_L23 the 1st glass lens (L1) The objective optical system which with the 2nd glass lens (L2) is smaller than distance d_L12, satisfies conditional expression d_L23/d_L12<0.25.物体側から順に、負のパワーをもつ第1レンズ(L1)、絞り(S)、正のパワーをもつ第2レンズ(L2)、第3レンズ(L3)からなり、第1レンズ(L1)の最も像側の面(2)が凹面、第2レンズ(L2)の最も像側の面(6)が非球面、第3レンズ(L3)の最も物体側の面(7)が凸面、で構成され、第2レンズ(L2)と第3レンズ(L3)の距離d_L23が第1レンズ(L1)と第2レンズ(L2)の距離d_L12よりも小さく、条件式 d_L23/d_L12<0.25 を満足する対物光学系。
