An image pickup optical system having an image pickup lens system having in turn a plurality of lenses arranged in multiple stages for forming an image of an object on an image plane of an image pickup device is disclosed. The image pickup optical system includes a double refraction plate arranged downstream of the image pickup lens system, a diaphragm unit, as a first optical unit, for determining the F-number of the image pickup lens system by a diaphragm aperture, and a second optical unit. When the F-number is equal to a pre-set value Fd or less, the second optical unit operates in conjunction with the double refraction plate for lowering the modulation transfer function (MTF) by the aberration due to an aspherical surface for a range of the spatial frequency not less than the cut-off frequency r.sub.c. Sufficient spurious signal suppressive effects may be obtained even if the optical low-pass filter is constituted by a sole double refraction plate, such as a quartz plate.