There is provided an objective optical system for endoscope with a lesser number of lenses and high-performance optical characteristics, equipped with a long back focus and a focusing function, and in which the aberration fluctuation accompanied by focusing is small. The objective optical system for endoscope includes in order from an object side, a first group G1 having a negative refractive power, a second group G2 having a positive refractive power, and a third group G3 having a positive refractive power, wherein the third group includes in order from the object side, a positive cemented lens CL2 and a positive lens L7, and observation from a normal observation state up to a close observation state is possible by moving the second group along the optical axis, and the objective optical system for endoscope satisfies the following conditional expression (1). 4 ‰¤ FB / f ‰¤ 7 where, FB denotes a back focus of the objective optical system for endoscope, and f denotes a focal length of the overall objective optical system for endoscope in the normal observation state.
