An optical element and a lens unit each includes: a liquid container transmitting light at least in a predetermined optical axis direction, the liquid container containing an insulating liquid and an ionic liquid being mutually immiscible and optically transparent, and having different refractive indices; a first electrode contacting the ionic liquid in the liquid container; and a second electrode insulated from the ionic liquid in the liquid container. The ionic fluid is not subject to electrolysis and does not evaporate easily even at high temperature. Accordingly, the optical element, the lens unit and an image taking apparatus having the optical element can maintain high light-transmission and optical characteristics over a long period even in a high temperature environment.