Abstract: In a projection exposure method of irradiating illumination light from an illumination light source onto a photomask made of a transparent substrate with a periodic mask pattern, and projecting the resultant transmitted light from the photomask on a wafer through a projection system, thereby forming an optical image of the mask pattern on the wafer, projection/exposure with respect to the wafer is performed by a main exposure operation and a sub-exposure operation to be performed after the main exposure operation. The main exposure operation is performed by irradiating the photomask with the illumination light whose coherency becomes 0.3 or less when a period L of the mask pattern on the image plane of the optical system is not more than a value obtained by doubling an exposure wavelength .lambda. of the illumination light source, and dividing the resultant value by a numerical aperture of the projection system.