an optical system consists essentially of , in order from the magnification side , a first lens having a positive refractive power with a convex surface on the magnification side , a second lens having a negative refractive power , a third lens having a negative refractive power , a fourth lens having a positive refractive power , a fifth lens having a positive refractive power , and a sixth lens having a positive refractive power , wherein the following conditional expressions and are satisfied : 0 . 17 & lt ; d 2 / f & lt ; 0 . 80 − 60 & lt ; ν d 1 − ν d 2 & lt ;− 15 where d2 : distance on the optical axis between the first lens and the second lens f : focal length of the entire system νd1 : abbe number of the first lens with reference to d - line νd2 : abbe number of the second lens with reference to d - line .