An objective optical system includes, in order from an object side, a negative-powered first lens (L1), a lens stop (S), a positive-powered second lens (L2), and a third lens (L3), wherein: a surface (2) of the first lens (L1) which is closest to an image side is a concave surface a surface (6) of the second lens (L2) which is closest to the image side is an aspheric surface a surface (7) of the third lens (L3) which is closest to the object side is a convex surface a distance d_L23 between the second lens (L2) and the third lens (L3) is smaller than a distance d_L12 between the first lens (L1) and the second lens (L2), satisfying a conditional expression d_L23/d_L12 <0.25.
