An imaging optical system suitable for imaging an object having a large spectral component at a certain spatial frequency. The imaging optical system includes an imaging lens system for forming an image of an object on a given image plane. The imaging lens system can have different magnifications without changing the positional relationship between the object and the image plane and includes a varifocal lens portion and a front lens portion arranged on the object side of the varifocal lens portion. The imaging lens system satisfies the following condition:.vertline..phi..sub.3T .vertline.-.vertline..phi..sub.3W .vertline..gtoreq.0.8.vertline.(Z-1)SA31.vertline.where Z is the ratio of the maximum value to the minimum value of the magnification of the imaging lens system; SA31 is the third-order coefficient of spherical abberation of the front lens portion; .vertline..phi..sub.3T .vertline. is the sum of the third-order coefficients of spherical abberation of the respective lens surfaces of the imaging lens system in the state of the maximum magnification; and .vertline..phi..sub.3W .vertline. is the sum of the third-order coefficients of spherical aberration of the respective lens surfaces of the imaging lens system in the state of the minimum magnification.In this imaging optical system, when the image magnification is varied, the magnitude of the spherical aberration caused by the imaging lens system varies with the magnification. Therefore, the variation of the spatial frequency response of the imaging lens corresponding to the variation of the image magnification is smaller than that of usual varifocal lenses so that the effect of spurious signal elimination remains unchanged even if the magnification is varied.