PROBLEM TO BE SOLVED: To provide an objective optical system having such characteristics that, as an objective optical system for 3D observation, when lenses are moved for focus adjustment, an adjustment stroke (space) can be ensured and parallax can be ensured and that as an objective optical system for 2D observation, the optical system can be reduced in size.SOLUTION: An objective optical system comprises, successively from an object side, a negative first lens having a concave surface at an image side, a negative second lens having a concave surface at an object side, a positive third lens, a brightness stop S, a positive fourth lens and a fifth lens that is a positive cemented lens, and satisfies conditional expressions (1) 2.5
