The oblique-viewing objective optical system includes a front-side lens group having a negative refractive power, an optical path converting element, an aperture stop, and a rear-side lens group having a positive refractive power. The front-side lens group includes a first lens and a second lens, and the rear-side lens group includes a third lens and a cemented lens which includes a positive biconvex lens and a meniscus-shape negative lens. The first lens is a negative lens having a concave surface directed toward an image-plane side, and the second lens is a single lens having a convex surface directed toward the image-plane side or a cemented lens, and the third lens is a positive lens. The following conditional expressions (1) to (3) are satisfied: 1.6