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<；LL1is/LsL2i<；1.25 ； ；(1)−；3<；(LL1is/RL1i)×(LsL2i/RL2i)<；−；1.25 ； ；(2)−；2<；fL22/(f×fL1)<；−；1.35 ； ；(3)2<；(nd1−；1.63)×(νd1−；31) ； ；(4)5<；(n2−；1.45)×(νd2−；31) ； ；(5)1.63<；nd1, and, 31<；νd1 ； ；(6)1.45<；nd2, and, 31<；νd2 ； ；(7).
