PURPOSE: To allow a finer outer diameter of a tip part of an endoscope without reducing the number of light guides by making the outer diameter of a lens at the tip part on the side of an object of an endoscope smaller than the outer diameter of a lens near an image pickup element arranged at the tip on the side of the object.

CONSTITUTION: An objective optical system of an endosocope comprises a lens group A at the tip part on the side of an object and a lens group B on the side of an image pickup element thereof. The outer diameter of the lens group A is smaller than the outer diameter of the lens group B. A tip adaptor lens AD with a small outer diameter is provided detachably on the side of the object in the objective optical system M built. An image of an object is taken by the adaptor lens AD so that light is admitted almost vertically into an image receiving surface 12 of the image pickup element arranged in the objective optical system M through a brightness stop S1. With such an arrangement, a telecentric system is built roughly. The optical system satisfies 1.4≤fA/fB≤7.2 and 2.3fA/fM≤10, wherein f represents the focal length of the lens group A, fB the focal length of the lens group B and fM a synthetic focal length of the objective optical system M.