An oblique-viewing objective optical system includes in order from an object side, a front-side lens group which includes a negative lens, an optical path converting element, an aperture stop, and a rear-side lens group having a positive refractive power, wherein the rear-side lens group includes a positive lens and a cemented lens having a positive refractive power, and the cemented lens includes in order from the object side a positive lens and a negative lens, and the following conditional expressions (1), (2), (3), (4), and (5) are satisfied:−2.0
