Objective optical system and endoscope using same

An objective optical system, comprising:

• a first lens group having a negative refractive power;
• an aperture stop;
• and
• a second lens group having a positive refractive power, provided in this order from an object side;
• the first lens group comprising a negative single lens and a cemented lens formed by cementing a positive lens and a negative lens together, provided in this order from the object side;
• the second lens group comprising a positive single lens and a cemented lens formed by cementing a positive lens and a negative lens together, provided in this order from the object side;
• and
• the objective optical system satisfying Conditional Formulae (1) and (5b) below:
• 15.0<νd(RN)<18.6 (1)
• −1.4<f1/f<−1.2 (5b)
• wherein νd(RN) is the Abbe's number of the negative lens in the cemented lens within the second lens group with respect to the d line, f1 is the focal length of the lens provided most toward the object side, and f is the focal length of the entire lens system.