Provided is an eyepiece optical system including, in order from an observation object (Ob): a first lens (L11) having positive refractive power; a second lens (L12) which has negative refractive power and of which an observation object (Ob) side lens surface is concave; and a third lens (L13) which has positive refractive power and of which an eye point (EP) side lens surface is convex. The third lens (L13) is fixed on the optical axis with respect to the observation object (Ob) when adjusting the diopter, and the following conditional expressions are satisfied: 0.65