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;
• the cemented lens within the first lens group being formed by a positive lens having a convex surface toward the image side and a negative lens, provided in this order from the object side;
• and
• the objective optical system simultaneously satisfying Conditional Formulae (1) and (2) below:
• −1.5<f123/f<−0.5 (1)
• 1.8<f456/f<2.1 (2)
• wherein f is the combined focal length of the entire lens system, f123 is the combined focal length of the first lens group, and f456 is the combined focal length of the second lens group.