The invention provides a wide-angle, low-cost endoscopic objective optical system that is reduced in terms of distortion and field curvature and composed only of spherical lenses, especially a video endoscopic objective optical system. The optical system comprises a first group G1 comprising a negative meniscus lens convex on an object side thereof, an aperture stop S, a second group G2 comprising a positive lens having a plane directed toward an object point side, a third group G3 including at least one concave refractive surface and having a positive refracting power as a whole, and a fourth group G4 comprising a cemented lens comprising a negative meniscus lens and a double-convex lens and having positive refracting power, so that an image is formed at an imaging device I via the first group G1 to the fourth group G4. A chief ray is reflected at the convex surface of the positive lens in the second group in a direction away from an optical axis.