In an exposing method which illuminates, by illumination light, a mask having apertures arranged in matrix and a latticed light shielding portion, projects the mask on an object to be projected via a projection optical system, and thus forms a dark portion pattern image at a position conjugate in relation to intersections of a lattice of the light shielding portion, an available light source shape of the illumination satisfies a specific condition. Therefore, it is possible to exposure a two-dimensional periodic pattern to a theoretical limit pitch being the same as that of a one-dimensional periodic pattern, and it is also possible to secure a depth of focus sufficiently.