In order to enable to sufficiently deflect the viewing direction for a distal end having an end surface substantially perpendicular to the optical axis of an objective optical system and minimize aberrations produced due to the deflecting action, the objective optical system for endoscopes comprises at least one surface designed as a deflecting surface or an eccentric surface on the image side of the surface nearest the object to be observed out of the surfaces of the optical components composing the objective optical system so that the light is refracted at least twice for deflecting the viewing direction. In order to shorten total length and minimize outside diameter of the objective optical system and enable to sufficiently eliminate the infrared light incident from the object, the objective optical system comprises at least one optical component made of a substance absorbing the light having the oscillation frequency of the YAG laser light and interference films reflecting the light having the oscillation frequency of the YAG laser light formed on the cemented surfaces of the optical components and/or the surface of emergence of a rear group designed as a positive lens component.