It is possible to ensure adequate depth in a range from normal observation to magnified observation, to provide a wide viewing angle during normal observation, and to easily perform focusing straightforward during magnified observation.Provided is an objective optical system comprising, in order from an object side to an image side: a first lens group (G1) having positive optical power a second lens group (G2) having negative optical power and a third lens group (G3) having positive optical power, wherein the first lens group includes, in order from the object side to the image side, a first lens (L1) having negative optical power and a second lens (L2) having positive optical power, and the second lens group is moved in response to a change in an object distance to perform focusing, and conditional expressions (1) and (2) below are satisfied: -19
