An endoscope observation optical system capable of improving the observation resolution of a mucous membrane of a living body is provided. The endoscope includes an observation optical system compatible with both standard visible-light observation and blood-vessel-enhancement observation, whose main component of a displayed image is an absorption peak of hemoglobin present in a waveband lower than 480 nm, wherein some longitudinal chromatic aberration remains in the endoscope observation optical system due to undercorrection at a wavelength of 415 nm, and wherein the focal position during blood-vessel-enhancement observation is set closer to a near point than the focal position during standard visible-light observation.