The invention relates to a pupil filter used for the illumination optical system of a semiconductor aligner or the like that can prevent a decrease in the quantity of light having transmitted through it, enhance the efficiency of semiconductor exposure, reduce loads of correction by the optical proximity effect and yield a stable yet high-resolution optical image without engendering size fluctuations of a pattern imaged on a wafer depending on a mask pattern pitch. Specifically, the invention provides a diffractive optical device for the formation of a pupil filter used for the illumination optical system of an aligner adapted to direct light emanating from a light source to a mask via an illumination optical system and project a pattern on the mask onto an alignment substrate and exposing it to light via a projection optical system. The pupil filter formed by the diffractive optical device is a dipole pupil comprising two light transmissive areas (11). The two light transmissive areas (11) are in a fan-form configuration symmetric at a given distance from the center of the pupil filter, between them there is an area (12) of low light transmittance, and outside the two light transmissive areas (11) and the area (12) of low light transmittance there is a light block area (13).