An imaging optical system having an image circle formed on an imaging element, the imaging optical system comprising: a plurality of lens elements arranged from an object side to an image plane side, and a diaphragm arranged between the plurality of lens elements, wherein the plurality of lens elements includes a freeform lens having a freeform surface that is asymmetrical with respect to a first direction and a second direction which cross with each other, wherein at least two freeform lenses are located on the object side of the diaphragm, wherein a diameter in the first direction is equal to or larger than a diameter in the second direction in the image circle, characterised in that the imaging optical system satisfies a following conditional expression based on a summation for freeform surfaces of freeform lenses located on the object side of the diaphragm: [Condition 1] ∑ k = 1 N SS k − SL K × Δ nd k YSH < 0 where N is a total number of the freeform surfaces of the freeform lenses located on the object side of the diaphragm, k is a number identifying a freeform surface among the total N freeform surfaces, SL k is a sag amount at a position where a height of a k-th freeform surface in the first direction is 50% of shortest image height, SS k is a sag amount at a position where a height of the k-th freeform surface in the second direction is 50% of the shortest image height, Δnd k is a difference resulting from subtracting a refractive index on the object side of the k-th freeform surface from a refractive index on the image plane side of the k-th freeform surface, and YSH is a height of 50% of the shortest image height. a plurality of lens elements arranged from an object side to an image plane side, and a diaphragm arranged between the plurality of lens elements, wherein the plurality of lens elements includes a freeform lens having a freeform surface that is asymmetrical with respect to a first direction and a second direction which cross with each other, wherein at least two freeform lenses are located on the object side of the diaphragm, wherein a diameter in the first direction is equal to or larger than a diameter in the second direction in the image circle, characterised in that the imaging optical system satisfies a following conditional expression based on a summation for freeform surfaces of freeform lenses located on the object side of the diaphragm: [Condition 1] ∑ k = 1 N SS k − SL K × Δ nd k YSH < 0 where N is a total number of the freeform surfaces of the freeform lenses located on the object side of the diaphragm, k is a number identifying a freeform surface among the total N freeform surfaces, SL k is a sag amount at a position where a height of a k-th freeform surface in the first direction is 50% of shortest image height, SS k is a sag amount at a position where a height of the k-th freeform surface in the second direction is 50% of the shortest image height, Δnd k is a difference resulting from subtracting a refractive index on the object side of the k-th freeform surface from a refractive index on the image plane side of the k-th freeform surface, and YSH is a height of 50% of the shortest image height. N is a total number of the freeform surfaces of the freeform lenses located on the object side of the diaphragm, k is a number identifying a freeform surface among the total N freeform surfaces, SL k is a sag amount at a position where a height of a k-th freeform surface in the first direction is 50% of shortest image height, SS k is a sag amount at a position where a height of the k-th freeform surface in the second direction is 50% of the shortest image height, Δnd k is a difference resulting from subtracting a refractive index on the object side of the k-th freeform surface from a refractive index on the image plane side of the k-th freeform surface, and YSH is a height of 50% of the shortest image height.