An endoscope optical system has a first lens group having positive refractive power, an aperture diaphragm, a second lens group having negative refractive power and a third lens group having positive refractive group, the second lens group moving along an optical axis to perform focusing, the first lens group having a first lens having negative refractive power and a second lens having positive refractive power, and the optical system satisfying the following conditional equation: - 5.0 < H 76 * 1 - nd �¢ 01 / r �¢ 2 < - 0.2. 0.5 < r �¢ 1 + r �¢ 2 / r �¢ 1 - r �¢ 2 < 1.2 where H(76) is a height at which a principal ray having an incidence angle of view of 76 degrees during normal observation passes through a surface on an image side of the first lens, nd01 is a refractive index of the first lens with respect to a line d, and r1 and r2 are respectively curvature radii of surfaces on the object side and the image side of the first lens.
