PROBLEM TO BE SOLVED: To secure a flange back even at a wide angle of view.SOLUTION: An endoscope objective lens optical system comprises, in order from an object side: a first lens group G1 that has a negative refractive power a second lens group G2 that has a positive refractive power a luminosity stop a third lens group G3 that has the positive refractive power and two reflection surfaces. The endoscope objective lens optical system satisfies following conditional expressions: Fb/FL>3.2...(1), d3/fl3<0.1...(2), and fl1/FL>-0.8...(3) where FL is denoted as a focal length, Fb is denoted as a distance from a final lens surface to an imaging surface, fl1 is denoted as a focal length of the first lens group, fl3 is denoted as a focal length of the third lens group, and d3 is denoted as a distance to the third lens group from the stop.
