An objective optical system includes a lens group and an optical path splitting element. The optical path splitting element is disposed on an optical path of the lens group, the optical path splitting element includes an optical path splitting surface configured to form a first optical path and a second optical path, an optical path length in the first optical path is different from an optical path length in the second optical path, a sum total of the number of transmissions of light and the number of reflections of light in the second optical path is larger than a sum total of the number of transmissions of light and the number of reflections of light in the first optical path, and an optical surface satisfying following conditional expression (1) is disposed on the first optical path, 0.8≤MR650/MR550≤0.9  (1).