Endoscope Optical System

An endoscope optical system consisting of 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 power, in this order from an object side to an image side,

• the second lens group moving along an optical axis to perform focusing, and
• the first lens group comprising a first lens having negative refractive power and a second lens having positive refractive power, in this order from the object side and satisfying the following conditional equations:
• −5.0<H(76)*(1−nd01)/r2<−0.2 (1)
• 0.5<(r1+r2)/(r1−r2)<1.2 (2)
• where H(76) is a height at which a principal ray of an incidence angle of view of 76 degrees during normal observation transmits through a surface on the 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 a surface on the object side and the surface on the image side of the first lens.