In order to enable a relatively narrow angle, a small outer diameter, a short total length, and the simultaneous correction of field curvature and coma aberration, provided is an endoscope objective optical system (1) which is provided with, in order from the object side, a first lens (2) configured from a negative single lens, a second lens (3) configured from a positive single lens, an aperture stop (4), a third lens (5) configured from a positive single lens, and a fourth lens (6) configured from a positive cemented lens, and satisfies the following conditional expression:(1) 0.3n1, (4) 0.6 n1, (4) 0,6 < IH/f < 0,83où Df est la distance entre la surface sur le côté objet de la première lentille et le diaphragme douverture, f est la longueur focale de lensemble du système, n1 est lindice de réfraction (ligne d) de la première lentille, n2 est lindice de réfraction (ligne d) de
