An objective optical system includes in order from an object side to an image side, a first lens having a negative refractive power, an aperture stop, and a second lens having a positive refractive power. An image-side surface of the first lens is a surface which is concave toward the image side, and an image-side surface of the second lens is a surface which is convex toward the image side, and following conditional expressions (1), (2), (3), (4), (5), (6), and (7) are satisfied: 0.6