Easier focus adjustment, making it brighter, wide angle and high definition image quality while making it finer. A front lens group (G 1) having a negative refractive power, an aperture stop (S) having a negative refractive power, and a positive rear lens group (G 2) in order from the object side, the front lens group consisting of a single lens of negative refractive power A third lens (L 3) which is a single lens having a positive refractive power and a second lens (L 2) which is a single lens having a positive refractive power, and a second lens , A cemented lens (CL1) of a fourth lens (L4) having a positive refracting power and a fifth lens (L5) having a negative refracting power and a sixth lens (L6) having a positive refracting power, An objective optical system (1) for an endoscope in which an object side surface of a lens is a flat surface, a second lens is a meniscus shape, a sixth lens is joined to an imaging element, and the following conditional expression is satisfied. 4
