An observation optical system for endoscopes configured so as to allow observation of objects located in liquids, comprising an objective lens system and relay lenses for relaying an image formed by the objective lens system, and satisfies the following conditions (1) and (2):0.08.ltoreq.N.sub.w .multidot.f tan [sin.sup.-1 {(1/N.sub.w) sin .theta..sub.Al }]/I.sub.1 .ltoreq.1.18 (1)0.9.ltoreq.N.sub.w .multidot.f.multidot.tan[sin.sup.-1 {(1/N.sub.w) sin .theta..sub.A0.5 }]/I.sub.0.5 .ltoreq.1.1 (2)wherein the reference symbol f represents a focal length of the objective lens system, the reference symbol N.sub.w designates a refractive index of a liquid, the reference symbols .theta..sub.Al and .theta..sub.A0.5 denote field angles at the maximum image height and a half of the maximum image height in water respectively, and the reference symbols I.sub.1 and I.sub.0.5 represent a maximum image height and a half of the maximum image height.