There is provided an oblique-viewing objective optical system having a high performance and a small size, and an endoscope for oblique-viewing using the same.The oblique-viewing objective optical system includes in order from an object side, 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 having a positive refractive power. 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. The third lens is a positive lens, and the cemented lens includes a positive lens which is a biconvex lens, and a negative lens having a meniscus shape, and the following conditional expressions (1) to (3) are satisfied: 1.6<；D1/f<；4.71.0<；D2/f<；3.3 and 9.0<；L/f<；31.0
