To off-axis and on-axis aberrations at low cost while having a wide angle of view, an endoscope objective optical system (1) has in order: a negative first lens (L1) a negative second lens (L2)and a third lens (L3) joined to each other a brightness diaphragm (S) and a positive lens group (G2)having a cemented lens (CL2) including one positive lens and one negative lens joined to each other, and satisfies expressions (1) and (2):where, D3(96deg) is a distance in which a chief ray of a d-line having an incident angle of 96 degrees that enters the first lens transmits the third lens, f_all is a focal distance of the whole system, r1 and r2 are radii of curvature of an object side surface of the first lens and an image side surface of the second lens, respectively.
