An objective optical system consists of, from an object side to an image side: a first lens group having positive optical power a second lens group having negative optical power and a third lens group having positive optical power, wherein the first lens group includes, from the object side to the image side, a first lens having negative optical power and a second lens having positive optical power, and the second lens group is moved according to a change in an object distance to perform focusing, and the following conditional expressions are satisfied:−19
