An ocular optical system configured to allow imaging rays from a display image to enter an observer's eye through the ocular optical system so as to form an image is provided. A direction toward the eye is an eye side, and a direction toward the display image is a display side. The ocular optical system sequentially includes a first and a second lens elements having refracting power from the eye side to the display side along an optical axis. Each lens element includes an eye-side surface and a display-side surface. An optical axis region of the eye-side surface of the first lens element is concave. An optical axis region of the eye-side surface of the second lens element is concave.