1. An exposure method comprising the steps of: forming a phase shift mask including a desired pattern and a cyclic dummy pattern overlaid onto the desired pattern, a part of the desired pattern to be resolved by effects of the dummy pattern being thicker than the line width of the dummy pattern; and illuminating the phase shift mask by using illumination light having a peak substantially on an optical axis in an intensity distribution to project light that has passed the phase shift mask onto an exposed plane via a projection optical system, thus transferring the desired pattern onto the exposed plane.