Provided is an eyepiece optical system including at least three lenses disposed in order from an observation object (Ob) along an optical axis. A final lens disposed closest to an eye point (EP) (corresponds to the third lens (L3) in FIG. 1) is fixed when adjusting the diopter, and the following conditional expressions (1) and (2) are satisfied: 2.2<|fe/fa|<6.0  (1) 0.5<|Re2/fa|<5.0  (2) where fe denotes a focal length of the final lens, fa denotes a focal length of the total eyepiece optical system (EL), and Re2 denotes a radius of curvature of the eye point (EP) side lens surface of the final lens. When an optical surface is aspherical, a paraxial radius of curvature is used for calculation.