Provided is an objective optical system capable of simultaneously performing narrow-band light observation and white light observation using a highly versatile imaging device while being a small optical system, and an endoscope apparatus including the objective optical system. The objective optical system 1 includes a lens group 2 that forms an image of an object, and an optical path splitting element 3 disposed on the image side of the lens group 2, and the optical path splitting element 3 includes a first optical path and a second optical path. The first optical path is formed on an extension line of the optical path of the lens group 2, and the second optical path is formed so as to intersect the first optical path. The optical image I1 in the second optical path and the optical image I2 in the second optical path are formed on the same plane, the reflecting surface is located in the second optical path, and one of the first optical path and the second optical path A predetermined optical surface is located only in the optical path, and the wavelength range of light transmitted through the predetermined optical surface, or the wavelength range of light reflected by the predetermined optical surface is limited, and the limited wavelength range is It is narrower than the wavelength range of the light traveling in the other optical path.小型な光学系でありながら、汎用性の高い撮像素子を用いて狭帯域光観察と白色光観察が同時に行える対物光学系及びそれを備えた内視鏡装置を提供する。 対物光学系１は、物体の像を形成するレンズ群２と、レンズ群２の像側に配置された光路分割素子３と、を有し、光路分割素子３は、第１の光路と第２の光路を形成する光路分割面を有し、第１の光路は、レンズ群２の光路の延長線上に形成され、第２の光路は、第１の光路と交差するように形成され、第１の光路における光学像Ｉ１と第２の光路における光学像Ｉ２は、同一平面上に形成され、第２の光路に、反射面が位置し、第１の光路と第２の光路の何れか一方の光路のみに、所定の光学面が位置し、所定の光学面を透過した光の波長域、又は、所定の光学面で反射された光の波長域は制限されており、制限された波長域は、他方の光路を進行する光の波長域よりも狭い。
