PROBLEM TO BE SOLVED: To provide an endoscope-purpose objective optical system that achieves miniaturization of an optical system and suppresses cracks in a lens.SOLUTION: An endoscope-purpose objective optical system comprises, in order from an object side,: a front group； a brightness diaphragm； and a rear group. The front group has an object side negative lens arranged on the most object side, in which an object side surface of the object side negative lens is a plane surface, and the rear group has, in order from the object side,: a positive lens； and a cemented lens. The cemented lens has: a double convex positive lens； and a negative lens. The endoscope-purpose objective optical system satisfies following conditional expressions (1), (2) and (3). 1≤φp2z/φp2a≤1.25 (1), 1×10≤Rzp2/φp2a≤9*10(2), and -10≤(fn*fc)/(fp1*fp2)≤-2.5 (3).SELECTED DRAWING: Figure 1
