While a wide angle of view , to reduce the diameter of while properly correct aberrations . In order from the object side to the image side, a first lens group (G1) having a positive refractive power , an aperture stop (S), the second has a negative second lens group having a refractive power (G2) and a positive refractive power and a third lens group (G3), the second lens group performs focusing by moving along the optical axis , the first lens group , the second has a negative refractive power in order from the object side 1 lens (L1), a positive second lens having a refractive power (L2), and provides an endoscope optical system satisfies the following conditional expression . -5.0
