There is provided an endoscope objective optical system with a small size and a high image quality, and in which, an adequate space for disposing a visual-field direction converting element is secured. The endoscope objective optical system, comprising in order from an object side:a first group having a negative refractive poweran aperture stop anda second group having a positive refractive power, whereinthe first group includes in order from the object side, a first lens having a negative refractive power and a visual-field direction converting member, andthe second group includes in order from the object side, a second lens having a biconvex shape and a cemented lens in which a third lens having a positive refractive power and a fourth lens having a negative refractive power are cemented in this order, andthe endoscope objective optical system satisfies the following conditional expressions (1), (2), (3), and (4)2.4≦d1/f≦4.6 (1)1.85≦f2/f≦2.6 (2)−50≦r21/r22≦−0.4 (3)2.01≦≦f03/f≦2.5 (4)where,d1 denotes an air conversion length from a surface on an image side of the first lens up to a surface of the aperture stop,f denotes a focal length of the overall endoscope objective optical system,f2 denotes a focal length of the second group,r21 denotes a radius of curvature of a surface on the object side of the second lens,r22 denotes a radius of curvature of a surface on the image side of the second lens, andf03 denotes a focal length of the third lens.
