Provided is an endoscope objective optical system in which aberration is not readily affected by manufacturing errors, aberration variation during focusing is reduced, and in addition, has a long back focus such that an optical member such as a polarising prism can be disposed therein. The endoscope objective optical system comprises, in the stated order from the object side to the image side: a front group (G1) which has a negative refractive power a focusing lens (F) and a rear group (G2) which has a positive refractive power. The front group and the rear group are permanently fixed on the optical axis, and the endoscope objective optical system satisfies conditional expressions (1) and (2): 4
