An imaging optical system includes an image forming optical system forming an image of an object, an imaging device placed in the proximity of the image of the object, a liquid crystal element placed at a preset position on the optical path between the object and the imaging device; and an optical element placed on the object side of the liquid crystal element. The optical element is constructed of a medium in which the transmittance of light at a wavelength of 500 nm is 20% or more and which exhibits different refractive indices with respect to light, the directions of vibrations of which are perpendicular to each other.