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 having a negative refractive power as a whole, an aperture stop and a rear lens group having a positive refractive power as a whole, wherein the front lens group includes in order from the object side, a first lens which is a single lens having a negative refractive power and a second lens which is a single lens having a positive refractive power, and the rear lens group includes a third lens which is a single lens having a positive refractive power and a cemented lens of a fourth lens having a positive refractive power and a fifth lens having a negative refractive power, and an object-side surface of the first lens is a flat surface, and the second lens has a meniscus shape having a convex surface directed toward an image side, and the third lens has a biconvex shape, and the objective optical system for endoscope satisfies the following conditional expressions (1) and (2)−2.0≦f1/Ih≦−1.39 (1)−0.2≦SF3≦0.61 (2)where,f1 denotes a focal length of the first lens,Ih denotes the maximum image height of the objective optical system for endoscope, andSF3 denotes a shape factor which is expressed by SF3=(R3L+R3R)/(R3L−R3R), when a radius of curvature of an object-side surface of the third lens is let to be R3L and a radius of curvature of an image-side surface of the third lens is lens to be R3R.
