Provided is an optical system for an endoscope that comprises, in this order from the object side, a negative lens group (G1), a brightness aperture (S), and a positive lens group (G2), where the negative lens group (G1) comprises, in this order from the object side, a negative first lens (L1) and a negative second lens (L2) the optical system for an endoscope is characterized by satisfying the following conditional formulas (1) and (2): 0.05
