The fluctuation of the angle of view due to focusing is reduced, and the variation of the observation depth is reduced while facilitating the focus adjustment. (G1) having a negative refractive power, a second lens group (G2) having a positive refractive power, and a third lens group (G3) having a positive refractive power in order from the object side, The second group (G 2) is provided movably in the optical axis direction and provides an endoscope objective optical system satisfying the following conditional expression. 9 2 / fa <16 (1) Here, f 2 is the focal length of the second group, and fa is the focal length of the entire system under the normal observation state.フォーカシングによる画角の変動を小さくし、ピント調整を容易としながら観察深度のばらつきを低減させる。 物体側から順に、負の屈折力を有する第1群(G1)と、正の屈折力を有する第2群(G2)と、正の屈折力を有する第3群(G3)とからなり、前記第2群(G2)が、光軸方向に移動可能に設けられ、以下の条件式を満足する内視鏡対物光学系を提供する。 9
