An objective optical system for endoscopes comprising a plurality of lens components and an aperture stop, and configured so as to permit adjusting a distance to an object to be observed simply by exchanging a lens component disposed in the vicinity of the aperture stop with another lens component having a focal length different from that of the lens component disposed in the vicinity of the aperture stop. This objective optical system for endoscopes can have an optimum depth of field by changing a diameter of the aperture stop along with the exchange of the lens component disposed in the vicinity of the aperture stop.