An objective optical system includes in order from an object side a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power, wherein focusing is carried out by moving the second lens group with respect to a change in an object-point distance, and the following conditional expressions (2) and (3) are satisfied:3<β (2), and60°<ω (3),where,β denotes a lateral magnification of the overall objective optical system at the time of focusing to an object point at a close distance, andω denotes the maximum half angle of view at the time of focusing to an object point at a long distance.
