There is provided a small-sized and bright objective optical system for endoscope with a wide angle of view and thin diameter, which enables to achieve a highly-defined image.The objective optical system for endoscope includes in order from an object side, a front lens group G1 having a negative refractive power, an aperture stop, and a rear lens group G2 having a positive refractive power, wherein the front lens group G1 includes in order from the object side, a first lens L1 which is a single lens having a negative refractive power and a second lens L2 which is a single lens having a positive refractive power, and the rear lens group G2 includes a third lens L3 which is a single lens having a positive refractive power and a cemented lens L45 of a fourth lens L4 having a positive refractive power and a fifth lens L5 having a negative refractive power, and an object-side surface of the first lens L1 is a flat surface, and the second lens L2 has a meniscus shape having a convex surface directed toward an image side, and the third lens L3 has a biconvex shape, and the objective optical system for endoscope satisfies the following conditional expression (1). −2.0≤f1/Ih≤−1.39 where,f1 denotes a focal length of the first lens, andIh denotes the maximum image height of the objective optical system for endoscope.
