A view angle is wide in the water, reliability as a medical endoscope is ensured, and mounting design is made easily. An endoscope objective optical system used for underwater observation and satisfying the following conditional expressions is provided: 1 < Iw / ft < 1.8 0.6 < Ia / Iw < 0.95 where Iw is a maximum image height in the underwater observation, ft is a focal distance of the entire objective optical system in observation in the air, and Ia is a maximum image height of a principal ray that is capable of being transmitted through an objective optical system during the observation in the air.
