An oblique-viewing objective optical system includes in order from an object side, a front-side lens group GF which includes a negative lens, an optical path converting element P, an aperture stop S, and a rear-side lens group GR having a positive refractive power, wherein the rear-side lens group GR includes a positive lens L2 and a cemented lens having a positive refractive power, and the cemented lens includes in order from the object side a positive lens L3 and a negative lens L4, and the following conditional expressions (1), (2), and (3) are satisfied: −2.0
