Small diameter conversion of the endoscope insertion section is assured, at the same time, wide angle of view is obtained. From object side the 1st lens which consists of the flat concave lens which directs the concave surface to image side the 2nd lens which consists of the flat concave lens which directs the concave surface to (2) and object side (3) and brightness contraction 5 and the 3rd lens which consists of the flat convex lens which directs plane surface to object side (6) it has for order, conditional expression below it offers the endoscope objective optical system 1 which it is satisfied (1) - (3). (1) 0.7fab/f0.9 (2) 1.7fe/fab2 (3) 2fb/fa4 Here, f: Focal length of all system, fa: Focal length of 1st lens 2, fb: Focal length of 2nd lens 3, fe: Focal length of 3rd lens 6, fab: It is synthetic focal length up to 2nd lens 3 from 1st lens 2.内視鏡挿入部の小径化を図り、かつ、広い画角を得る。物体側から順に、像側に凹面を向けた平凹レンズからなる第1レンズ(2)と、物体側に凹面を向けた平凹レンズからなる第2レンズ(3)と、明るさ絞り5と、物体側に平面を向けた平凸レンズからなる第3レンズ(6)とを備え、以下の条件式(1)~(3)を満足する内視鏡対物光学系1を提供する。(1) 0.7≦|fab/f|≦0.9(2) 1.7≦|fe/fab|≦2(3) 2≦fb/fa≦4 ここで、f:全系の焦点距離、fa:第1レンズ2の焦点距離、fb:第2レンズ3の焦点距離、fe:第3レンズ6の焦点距離、fab:第1レンズ2から第2レンズ3までの合成焦点距離である。
