An imaging optical system has an image circle defined on an imaging element. The imaging optical system includes: lens elements arranged from an object side to an image plane side, and a diaphragm arranged between two of the lens elements. The lens elements include freeform lens elements, each having a freeform surface that is an asymmetrical surface with respect to a first cross section and a second cross section, the first cross section being defined by a first direction and an optical axis direction, the second cross section being defined by a second direction and the optical axis direction, the first direction crossing the second direction, and the optical axis direction being orthogonal to the first direction and the second direction. At least two of the freeform lens elements are located on an object side of the diaphragm.