PROBLEM TO BE SOLVED: To solve the problem in which it is difficult to design an objective optical system that is small and has a wide angle of view while keeping optical performance excellent.SOLUTION: An objective optical system for an endoscope is composed of, in order from an object side, a first lens group having negative power and a second lens group having positive power. The first lens group is composed of, in order from the object side, a negative lens training a concave surface on an image side and a positive lens training a convex surface on the image side. The second lens group at least includes, in order from the object side, a positive lens training a convex surface on the image side and a doublet composed of a negative lens and a positive lens joined together. The objective optical system satisfies prescribed conditions.SELECTED DRAWING: Figure 2COPYRIGHT: (C)2018,JPO&INPIT【課題】光学性能を良好に維持しつつ小型かつ広画角に設計することが難しい。【解決手段】物体側から順に、負のパワーを持つ第一のレンズ群、正のパワーを持つ第二のレンズ群からなり、第一のレンズ群が、物体側から順に、像側に凹面を向けた負レンズ、像側に凸面を向けた正レンズからなり、第二のレンズ群が、物体側から順に、像側に凸面を向けた正レンズ、負のレンズと正のレンズとを接合した接合レンズを少なくとも有し、所定の条件を満たす、内視鏡用対物光学系を提供する。【選択図】図2
