While being small in size and diameter and allowing a wide angle of view, an endoscope objective optical system has a simply-structured frame for holding a lens and inhibits the occurrence of flare caused when rays are reflected at an inner circumferential surface of the frame. Provided is an endoscope objective optical system (100) including, in order from an object side, an aperture stop (AS), a front group having positive refractive power (GF), and a rear group having positive refractive power (GR), in which the rear group (GR) is formed by joining a single lens (LP) having positive refractive power and a single lens (LN) having negative refractive power and is joined to an imaging device and a joining surface (Sc) between the single lenses (LP and LN) has positive refractive power.
