The diameter of an endoscope insertion portion is reduced, and a wide field angle of an endoscope insertion portion is obtained. An endoscope objective optical system includes, from an object side, a first lens formed of a plano-concave lens having a concave surface facing an image side, a second lens formed of a plano-concave lens having a concave surface facing the object side, an aperture stop, and a third lens formed of a plano-convex lens having a plane surface facing the object side, and satisfies 0.7 ‰¤ fab / f ‰¤ 0.9 1.7 ‰¤ fe / fab ‰¤ 2 2 ‰¤ fb / fa ‰¤ 4 where f: focal length of the entire system fa: focal length of the first lens fb: focal length of the second lens fe: represents focal length of the third lens fab: ombined focal length from the first lens to the second lens.
