Provided is an objective optical system that is not easily affected by various errors and in which various aberrations are well corrected. The objective optical system includes, in order from the object side, a first lens group G1 having a positive refractive power, a second lens group G2 having a negative refractive power, and a third lens group G3 having a positive refractive power. Focusing is performed by moving the group G2, and the third lens group G3 includes at least a positive lens L7 and a cemented lens CL3. The cemented lens CL3 of the third lens group G3 includes a positive lens L8 and a negative lens. L9, the first sub-lens group SUB1 of the third lens group G3 is located closer to the object side than the predetermined air interval, and the second sub-lens group SUB2 of the third lens group G3 is predetermined air The predetermined air gap is located on the image side of the gap and is the maximum air gap among the air gaps in the third lens group G3, and satisfies the following conditional expressions (1) and (2). 1.21 <；fp / f <；2.42 (1) 0.35 <；tt / f <；0.6 (2)様々な誤差による影響を受けにくく、諸収差が良好に補正された対物光学系を提供する。 対物光学系は、物体側から順に、正屈折力の第１レンズ群Ｇ１と、負屈折力の第２レンズ群Ｇ２と、正屈折力の第３レンズ群Ｇ３と、で構成され、第２レンズ群Ｇ２を動かすことにより合焦を行い、第３レンズ群Ｇ３は、正レンズＬ７と、接合レンズＣＬ３と、を少なくとも含み、第３レンズ群Ｇ３の接合レンズＣＬ３は、正レンズＬ８と、負レンズＬ９と、を有し、第３レンズ群Ｇ３の第１副レンズ群ＳＵＢ１は、所定の空気間隔よりも物体側に位置し、第３レンズ群Ｇ３の第２副レンズ群ＳＵＢ２は、所定の空気間隔よりも像側に位置し、所定の空気間隔は、第３レンズ群Ｇ３における空気間隔のなかで最大の空気間隔であり、以下の条件式（１）、（２）を満足する。 １．２１＜ｆp／ｆ＜２．４２ （１） ０．３５＜ｔt／ｆ＜０．６ （２）
