ENDOSCOPE OPTICAL SYSTEM

The purpose of the present invention is to provide an endoscope optical system capable of switching between normal observation and magnified observation, and having satisfactorily corrected chromatic aberration. An endoscope optical system comprises, in order from the object side, a first lens group G1 having negative refractive power, a second lens group G2 having positive refractive power, and a third lens group G3 having positive refractive power, wherein: switching between normal observation and magnified observation is possible by securing the first lens group G1 and the third lens group G3 and moving the second lens group G2; the third lens group G3 has a cemented lens; the cemented lens has an object-side lens and an image-side lens; and the endoscope optical system satisfies conditional expressions (1), (2). 0.471 < nd02 - nd01 < 0.475 (1) 52.6 < νd01 - νd02 < 53 (2)