Provided is an observation optical system used for observing an image displayed on an image display surface. The observation optical system includes, in order from an observation surface side to the image display surface side, a first lens having a positive refractive power, and a second lens having a positive refractive power. The first lens is
a Fresnel lens, and a focal length f1 of the first lens and a focal length f2 of the second lens are each appropriately set.