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):1.0
