short outer diameter of the full-length small in a relatively narrow angle, and, curvature of field At the same time it wants to correct a coma with. In order from the object side, a first lens made of a negative single lens (2), a positive second lens composed of a single lens (3), an aperture stop (4), a third lens made of a positive single lens and (5), and a fourth lens composed of a positive cemented lens (6), to provide an objective optical system (1) for endoscope that satisfies the following conditional expression. (1) 0.3 n1, (4) 0.6 n1、(4)0.6
