An objective optical system for endoscopes has, in order form the object side, a single lens with negative power, a cemented lens with positive power, and an image sensor unit. The cemented lens has a plano-convex lens, located at the image-side end, with a convex surface facing the image side and an aperture stop placed proximate to a cemented portion and the image sensor unit includes an optically cemented body of at least one optical part, a path bending prism, and a solid-state image sensor to satisfy the following conditions: 1.5